wpa_supplicant doesn't affect modem (2G/3G) in any way.

you can use wpa_supplicant on N900 (if you compile fixed version) just fine. However there are a few caveats in connection with how N900 handles connections and notifying applications of the availability of a connection.

Long story short: you need to stop wlancond so that wpa_supplicant can access the wlan0 interface. then you also need to use a 'dummy' network connection on the connection selection ui.

also, you need to do your own power management