Most of the guys that I know that use a guitar with their keyboard run their guitar and mic through a small, Alesis mixer, then directly into the keyboard's mic/line input and switch it to line. For the most part, this tends to work out pretty darned good. The mixer sells for about $100 and also has effects.

Good luck,

Gary cool
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!

K+E=W (Knowledge Plus Experience = Wisdom.)