You'd need to make an amplifier circuit. At the minimum, something along the lines of a variable gain diffence amp for each input, then feeding those into a summing amp.
In case you have no idea what I'm talking about, National Semiconductor has some good app-notes for their op-amps. Start with
AN-20 "An Applications Guide for Op Amps".
If you value your time and audio quality, buying something professionally made is best.