Resources (Skins, Objects, etc) from the PC version of The Sims appear to be genereally compatible with the Mac version. One annoying problem is that some of the filenames exceed the Macintosh's limit (31 characters). Presumably, one could change the filenames to shorter names, but I am concerned that some files may reference data in other files, and changing names would perhaps break those links.
The Sims uses a special file type (.far) as a container for multiple files. Each entry in the .far file has an "entry name" (basically the original filename of the file added to the .far file). Although the .far file itself is still subject to the MacOS filename lenth limit, the entries within that file can have arbitrarily long names.
I created a special utility, FarMaker, to create .far files from a folder of individual files. There is also a companion utility, Unzipper, which expands ZIP files and maintains any long filenames in the comments field of the Finder information. FarMaker then makes use of these comments to ensure that the .far file has the full entry names rather than filenames that have been truncated to the Macintosh's 31 character limit.
Both utilities are freeware and have been only minimally tested. They don't even have icons yet!
Download FarMaker and Unzipper 0.4 (144K)
Several people have asked about a tool that does the reverse of FarMaker. I don't have any plans on writing one, but someone else already has a tool that is pretty similar...check out FarSight by Joe Strout.
Feel free to download the source code. FarMaker and Unzipper were written in C++ using Metrowerks Code Warrior (you'll ned verion 8.0 to read the project files). Comments are pretty minimal. To build Unzipper you'll also need a copy of the zlib library, which is freely available at other web sites.