Is it possible to use combined network connection using (slow) DSL and wireless connection on, e.g. Windows 7, so the resulting network transfer rate can be improved?
Only one connection will be used. You may want to look in to proper load balancing / router with a 3G/USB stick socket.
It isn't possible to tie the connections together without a special router or support from the ISP, however you can do round robin or fail over.