If you want to send nurbs objects to XSI, you'll have to use the iges format. But beware that it's very likely that you will get problems with trims.
If you want to send polygons, then use the obj format. In the Net View you have an obj importer/exporter add-on that you can install in XSI.
The other option is to use the dotXSI exporter for Maya, that you can get here:
http://www.xsibase.com/cgi-bin/yabb/YaBB.c...;num=1033064788
Hope this helps
Salutations - Cheers
Bernard Lebel