Yes you can. Many folk use VG to dilute liquids which are PG-based to start off with. The only thing you may want to watch out for is the glycerine may make your liquid a bit too thick and viscous to settle into cartridge wadding properly. If it does you can add purified water to thin it back again or use vodka like I have!
In fact I've found adding a bit of VG to Johnson Creek
juice makes it a better consistency as I found the JC alone slightly too thin. There's lots more advice and recipes in the long 'Lorann flavours'
thread. Have fun!
Have fun!