If you model in 3ds max or maya you can port the models to XSI through the FBX format. Make note, however, that FBX/XSI renames dashes in the names of objects/helpers to underscores when you use this method if they were precreated/named in said programs, so you'll need to fix those once converted to mesh (using replace in notepad++ makes this a 10 second job).
