Can I use some of my SSD memory as RAM?

The MacBook Air already uses the SSD as a sort of RAM, when it manages it as Virtual Memory. It is far from a substitute though. It is far, far slower in access, and Read & write speed. Moreover, it's use should be minimized when possible. This is because the SSD's NAND cells have a finite write capability that RAM doesn't. While it is not an issue for occasional use, massive numbers of repeated writes can eventually take a toll. It's not going to ruin your SSD right away, and there are scant few reports of SSD problems at all. But if the time ever comes, it is fortunate that the SSD is on a removable board.

4 GB of base memory suffices TODAY for most users, but for those who do need more, configuring to 8 GB is the most reaonable solution. Unfortunately, the MBA is not designed to be upgradeable. As such, users of the MBA had best choose a configuration at time of purchase that best meets their needs. If you think you need to upgrade later, then that is reason enough to consider one of the MacBook models that afford memory upgrading for users.

There are come new schemes which what they are calling Levelling. Instead of just writing to the same blocks over and over they spread it around. Kind of a good idea. Seems I read that the life span is projected at 10M operations.

Basically you already have. The OS and apps run in virtual memory that can exceed the size of real RAM installed. The SSD is used to store the excess pages.