Unclear on the concept....
If you plug your guitar in through and audio input, you cannot control software instuments from it. Real Instuments will simply record the sound of your guitar.
You would either need a guitar with a hex pickup and a MIDI interface, or you would be looking for some kind of physical-modelling effects box that would take the audio from the guitar and severely modify it to resemble a banjo.
You might, just maybe, sorta get something that sounds a bit like a banjo by using and envelope follower effect with some major filtering, but that would be for single note lines only.