You probably already have that inside: http://www.rmcpickup.com/polydrivei.html
It act as the 6 string buffer and a piezo preamp
RMC offers the option "saddle mix" a piezo bus to your existing output , just have to add a switch or blend pot and a little tweak inside.
If you use the RMC piezo internal preamp , no need for a special DI , just your normal 1/4 out and blend RMC and mag signals to your liking.
There a lots of possibilities with GK systems…..