I strongly suggest you leave your preamp alone and instead construct an additional preamp for the piezo pickup.

It will need to be a very high input impedance - more than 1 meg ohm but the sensitivity doesn't need to be high as the output of a piezo cartridge is typically 500 mv to 1100 mv, almost enough to produce a usable output from the amplifier directly into the line in...

The gain of the piezo preamp need not be great, merely enough to provide enough drive to match the line input, but it should provide RIAA correction - google will find plenty of active and passive circuits for this.
The current required for such a simple circuit is so small battery power would be a good choice for simplicity as it will simplify screening and cut hum and other interference to a minimum.