The CMDB has a lot of OOTB classes that you might be able to fit those items into. If none of them meet you needs you can simply create you own class or extend an existing one, which sounds like your best bet. Perhaps Software would be a good place to extend from. 

To get the actual assets into ServiceNow you can do integrations into systems where the assets are already uploaded into or you can do uploads with spreadsheets. I've had success with a couple of different methods of uploading spreadsheets that include via manual upload, email, and service request.

Hope this helps!

-Joel R.