Can I add a patio sealer to my polymeric sand patio?
It's normal for it to loosen a bit the first season or two as things finish settling. If you're in a freeze/thaw cycle climate that can also cause that. 

You should be able to apply your sealer over the sanded joints, though it may not prevent this issue from happening again. Patios take maintenance. I had to re-apply a new layer of sand every few years to fill in the gaps here and there.