What file format are they? So long as you can import a model into Blender or Max, you can convert any user-made mesh into the .nif file format.
I remember i played with sims 2 mod hairs some time ago, i used a program called SimPE it can edit/pack/unpack sims 2 package files so i could export modes as OBJ's,
check www.modthsims2.com , i think they have some programs in the downloads
Would be nice to get some hair meshes from sims2 moders
, o yea and dont try to convert any stuff you need actually
pay for (yes there are sites that are charging cash for mods!)