As far as we know, it's not possible to combine an Enterprise and National loyalty account together or move points between them. You're probably much better off crediting future Enterprise rentals to your National Emerald Club account but adding that number to your Enterprise rentals. It's a superior program overall, but only if you can make use of the National credits.

Note that National is at all major airports, and many smaller ones, and they do have some off-airport locations, but not nearly as much off-airport coverage as Enterprise does, so if you mostly rent off-airport then our advice would probably be different.