Can I Share System Memory with GFX Card?

Obviously no.

 

Obviously you can't use the system memory to extend the RAM of the graphics card, as these use different technologies and are physically connected in a completely different way. Nevertheless, the system memory is automatically used for caching etc. so the memory is being "shared" in a way.

All that besides, 512Mb should be more than enough for a flight simulator like X-plane, even on the native 27" resolution of the iMac. Judging from the screenshots, I very much doubt that it uses high-resolution multi-layer textures or other stuff modern games use which eats lots of GPU RAM. So if you have performance problems, I doubt that increasing the GRU RAM will help.