As I read it, as a portable miniature wall of force, the ring gives the wearer a bonus of +2 to her AC and functions like a shield. This is not the same as giving the wearer a +2 shield bonus on her AC.
I'd refrain from "enhancing" the force shield ring.
Short answer, no. It's not a shield, and cannot be enhanced like a shield.
Longer answer, yes... you should be able to create a magical (force) shield of greater than +2, and there are simple rules to that effect. I'd impose a limit of +7, to mimic a magical large shield. You simply have to start with a shield "bonus" of +0, which is the normal non enhanced armor bonus to a force shield with no magic... (nonexistant), and then subtract the cost for a +2 (force) shield and add in the cost for whichever + you are going for, just like normal magic item creation.