As has been said before, gsm is in no way compatible with cdma.

You can try using your usb modem with the n900 via h-e-n. You will need to provide external power for the modem as n900 will not provide enough drive for it.

I'm not sure how to solve the issue of zerocd/modeswitch as some cdma modems depend on it. If you are lucky, your modem will just be recognized. All that will be left is to set up the rules for the data connection.