I dont know if milkshape has a .smd exporter (which is what you need) but any model in theroy can be put into any other game. Only the problem is that UV maps sometimes cant make the software crossover from model tool to the next.
IF you really want to, look up how to convert milkshape models to XSI format, or use XSI to export the Sims models (Assuming you know how to decompile them, and then export them)
Anyway, Sims objects are low poly and could be modeled in like 2 seconds, UV mapped in like 10 minutes XD