Niol (and the others who have helped research this), thank you so much for this info. I've been dabbling at it and I came up with these instructions for myself as to how to quickly change a 1x1 non-bottomful regular floor straight out of Homecrafter into a properly-working bottomful. Your instructions are lovely, but a little bit hard to follow, and I wanted to just post up some little instructions that are exactly what you need to change, start to finish.
1. Open your finished floor tile in SimPE.
2. Click on the Material Definition (TXMT) resource in the top left pane, then again in the right pane.
3. In Plugin View, click on the cMaterialDefinition tab, and under Type, change "Floor" to StandardMaterial.
4. Still in Plugin View, click on the Properties tab.
5. Now you need to add two values. Click add twice. This will add two lines to your list of items in the left box. Click on the first : displaying there.
6. Over to the right, in the first box, type "stdMatCullMode" and in the box below type "none".
7. Now click on the second item that you added, the remaining : symbol. Again in the first box, type "stdMatLightingEnabled" and then in the box below type "0" - that's a zero, not the letter o.
Now below that, click sort List. This will put all the items within in the right order.
8. Click on the line starting with stdMatDiffCoef. Change the value for it to "1,1,1,1".
9. Click Commit, then OK. Then File - Save.
These instructions are just for single-tile floors - 2x2 and 5x5 floors are probably more complicated, but I wanted to just basic, quick instructions.
Question though - my floors are bright and light and look just like the texture I have used, but only in bright sunlight - bottomful floors on any sort of interior are quite a bit darker than they should be. Is this due to me screwing up the above or missing a step? They look great if I leave the roof off but otherwise are MUCH darker, even when fully lit by interior lighting.