How should I add flooring in my "crawl" space?
Darvis,

I would build a deck out of wood and plywood. Use treated wood if you think there is ANY chance of moisture or insect issues.

Support your deck beams with short piers of their own. Then just cut the plywood to fit around the existing peirs. I don't see a need to tie the new deck into the existing posts. It doesn't sound like the new deck can pull away from a wall or fall over.

Whatever you do you might want to put down a vapor barrier and insulation.