I used to play SL but havnt recently, so things might have changed a bit. I used to put things together in Wings3d and then play around with the exported maps in gimp or photoshop.
Blender has some plugins that might do this also- check out the second link below.
To save you using all your L$ when uploading to SL there is ( or was) a beta testing grid that you can login to when entering SL. You can test your maps there before uploading to SL.