Here ya guys!
After reading the thread by ChanG i tried to replace the default nude skin for normal light skinned female and... uhu, i had success! :-)
Guess i can change any tex i like with this method, but it's still such a *tricky thing* and any suggestion is welcome to make it smoother.
Unfortunately, due to filename mismatch into bodyshop packages, you can edit only ONE textures at a time: You will see later why... (I said it's
tricky ehe:-) ). Any suggestion to fix this annoy is hardly wanted.

What tou need:
Game extractor v. 1.05 (do not use newer version as they don't show filenames into packages).
A batch image processor: i used Thumbs Plus as it's easy and powerful. (not mandatory, but it will make things easier...:-P)
Any image editor you like.

NOTE: As this tutorial refers to a non-english version of the software, items name may change slightly.

Step 1: Run bodyShop and choose create body parts/genetics/skintones

Step 2: Export a skin tone, naming the project with something intuitive (s. as light - tan - medium etc.)

Step 4: Go to your projects folder, find the texture you want to mod (f. e. afbodynormal~top~stdMatBaseTextureName.bmp) and make any modification you like, being sure not to keep file properties and name, then save it overwriting. (you brobably will have to quit Bodyshop as the file could be in use).

Step 4b: And here is the tricky part: into your project folder you will find a file called filename.package (where "filename" is the project name you have saved with):
This file contains the .txtr file you need to replace the default skin with, but when you open the package with game extractor you can't find the match, as the filenames
of the .txtr files are just a bunch of meaningless characters.
I have found a "funny" way to easily recognize the file ino the package: before re-importing into bodyshop i replace all other images in the project folder with dummy files! you need to batch-process all these images (except obviously the modded texture) to make them all black or all white, as bodyshop seems to auto-detect
identical file when exporting, and merge them to one single image!

Step 5: So, re-open the modded project into bodyshop and import into game (you will notice the mannequin head is completely black or white), then go back to "create body parts", select the new texture (there should be a black or white thumbnail) and export it again. (you can delete the previous exported project as you don't need it anymore).

Step 6: Now run game extractor and open the "projectname.package" into your project folder (where "projectname" is still the name you saved the project with).
Sort by file type and look for the .txtr files: there should be just a few ones (2 or 3 files), and one of them should be definitely larger in size than the other ones
(around 200k against 0-1k) That file is the texture you need. So export the file into a folder on your PC.
Now open sims07.package (located into C:\Programmi\The Sims 2\TSData\Res\Sims3D) with game extractor, find the .txtr file which correspond to the modded texture*
and press "replace file with one on your computer": of course you will replace it with the new texture you exported before, so go to the proper folder into the dialog box and
select it, then open and replace. (NOTE: you will find four files for it, the only one you need is the one with extension .txtra , should be around 200k).

Step 7: backup sims07.package then save the new one to C:\Programmi\The Sims 2\TSData\Res\Sims3D. DONE! you can now test the new texture into the game.

*Filename guide:

Again, filenames into the sims07.package don't match the name of the edited texture (i hate Maxis), so you need some tips to find the exact match:
Filenames come into the following format (example): afbodynaked-nudesoft-s1.txtr: In this case af means adult, f means female, bodynaked and nude mean just what they say,
soft means fat and s1 means light skin.

Here is a quick guide to filenames:

a = adult
t = teen
e = elder
b = baby
c = kid

f = female
m = male

soft = fat
cut = muscular
(nothing) = normal

s1 = light
s2 = tanned
s3 = medium - dark tanned
s 4 = black

Hope you have fun, bye! ;-)

Well, it works, sort of. I don't know if this is everyone but did you test it? the game will crash to desktop after 5-20 minutes of play.

You should probably go back and read that full thread. It is easier to find the corresponding txtr files by looking in the txmt files for the names comparisons, you don't have to find them one (or I was doing early on) four at a time. Also, I guarantee this will crash after 5-30 minutes, we have already gone through all of this and you don't have to even change or modify your existing sims07.package, you can do it with an external package in the downloads directory. Chang and I have got it running reliably for up to 1 hour, but it will still crash. there is some data created in bodyshop created txtr files we have to still figure out.

Thanks for the hints, so the game crashes??? i tested it just for a few minutes, it seems to work fine but i guess i must try it a bit longer.
If the crash comes i will try to discover the cause, tahnk you again.

As near as I've found, the crash (at least in my games) was always related to a service vehicle arriving (car pool, maid, gardener...)

Not sure if that's reliable enough in other people's experience. I was using Duddly's test skins, btw. As soon as I removed them, blammo - crash vanished!

It made me wonder - is there a special reference for the driver of the carpool or something?