Can I volunteer to work with my Masters thesis advisor, when I am funded by different advisor for PhD
Your personal time is yours by definition. A good idea though might be to talk to your
Ph.D. advisor. If your former and current research projects do not overlap to much and you do not use results obtained in one lab and not yet published in the other lab I don't see any problem. Of course there may also be some personal animosity or rivalry between the former and current advisor so its a very good idea that your Ph.D. advisor is aware of this other activity and does not find out about it from a third party.