Have breast expansions filled to 360. Doctor wants to stop expansion due to thin skin. Can I ask for more and medically be safe?

It is up to the judgment of your plastic surgeon. Based on many factors including skin quality, body proportions, etc. your surgeon's advice is generally best to follow. You can always seek a second opinion from an experienced board certified plastic surgeon but your own surgeon knows you best. Good luck. 

I would trust your doctor if they advice against further expansion if your skin is too thin. It would be a better choice to be slightly smaller than to have the a open wound,infection and loss of the entire reconstruction.It is also possible to place a larger implant into the pocket than the actual volume in the expander. Nowadays with shaped implants, it is normal to underfill an expander and to place a larger implant during the second surgery. Discuss your concerns with your surgeon and he/she can guide you. 

I would trust your surgeon's judgement. The last thing you want is either skin necrosis or tissue expander exposure.  Best of luck.