So This may be a bit of a hard question so please bear with me, So I recently downloaded a mod which extends the lifespans of sims drastically, however before I discovered this mod I had households created before and dumb me thought that adding the mod would automatically add days onto every sims lifespan but it didn't, My question is, Is there any possible way I can get the sims who are currently on the maxis lifespan on board with the mod lifespan?

There's Neighborhood Age Correct. I'm not sure if I can directly link to the source but here's a link to a link: http://modthesims.info/showthread.p...25#startcomment

If you don't currently have an account at BAS (primarily an adult site), however, I'm not sure if you can sign up for one at the moment. They seem to be having registration problems.

In the meantime, you can use other ways to work with aging. Townies don't age by default until they're made playable so you can leave them as is for a while. You could keep track of the sims' actual ages and:
1) manually adjust playables' ages in SimPE 2) use the Elixir of Life to extend their days
3) mods: SimBlender to adjust age (3 days longer/shorter, reset to Max days - but this won't set them to the new age range) and others
4) Use cheats/mods to age down playables and age them back up again. It'll give them the proper lifespan this time. (Your profile doesn't indicate you have FT, which introduces an option to age townies up with your sim, but they'll age up with the new life span as well.) This might be your best option if you don't have access to BAS or SimPE. Anyone who ages up in-game will automatically adhere to the new lifespan, so you can just wait for the younger ones if you want to.

Edit: Ugh, the format looks fine when I edit it myself but it doesn't show up properly in the actual post.

Edit 2: I found a link to Neighborhood Age Correct: https://app.box.com/s/6hzj2igaefj94eixehm17up2ubug6qqv

I just make note of how many days I need to add or remove from a sim and then use the sim blender or sim manipulator to add/remove days. Most of my adult sims start out needing I think 26 extra days, so it takes a while of playing as them before that's actually possible.