Humph, Masked F: any
putty (or clay) can be mixed together.
Currently working on something with an odd mix of tamiya quick type, apoxie sculpt, and terracotta milliput.
Personally I wouldn't add more than 1/5 - 1/4 milliput to begin with. It can be somewhat overpowering stuff, especially if you want to preserve the feel of the GS. Magic sculp or apoxie sculpt create less of a shift for the same amount, IMO, and as per Freak's advice; though it's not a huge difference. See here
for stuff about mixing GS and apoxie sculpt. Although with the proportions he uses, 'course it's going to affect the GS properties. In that's case I'd say it's more about binding the AS than tweaking the green.
In my experience green stuff is the only one you have to worry about going off. Milliput and apoxie sculpt are definitely easier and better to use fresh (bit of a shock when you finish an old packet and start a new one) and can go a bit stiff and develop a yellow skin after a while, but they're still perfectly usable after mixing. Ditto with old bars brown stuff and that tamiya quick type, that've gone kind of hard. YMMV. There's also (or used to be) a strange problem with magic sculp where one half would crystallise and harden. Happened with mine, but I followed some advice and a bit of heat brought it back to life.
Mixing milliput, magic sculp etc. etc. into GS also counteracts the negative effects of the latter going stale.