I am just curious about the structure of that hook at the wrist... Is it like a panda's thumb and can be moved independently of the hand?

It's not actually a bone; apparently it's called the styliform cartilage, an addition to the wrist which can be moved to erect the patagium.

 

http://si-pddr.si.edu/dspace/bitstream/10088/4761/1/VZ_rwt5.pdf

so the squirrel can fold back this bone at will?

 

so can we imagine when the time and ecology pressures are right, maybe this bone might become a longer until this creature can fly?

http://www.jstor.org/pss/1382860

 

Apparently, it can be both erected and folded back voluntarily.

Wow imagine what this creature will become in the future..

 

A real "Flying" Squirrel once that piece of bone becomes longer.