I've had no problems putting weight in the buttcaps of my Six.One's. I use tungsten putty, but I've used lead tape for other racquets.
You can also dig out some of the foam that's inside the racquet handle and place weight there.
USRSA MRT - Gamma 6004 + Wise 2086
K Six.One 95 16x18 - Pacific Prime Gut/Genesis Black Magic