Hey Krista,
The global voucher data is stored in the headquarters database in the voucher and voucheronline tables. I think your best bet would be to set up global vouchers as if you were doing a new implementation, then export your old data into a csv file, perform a bulk insert into a temporary table, and then transfer the information from there into the two respective tables. This will take a little bit of SQL know-how to pull off.
I will caution, though, that my experience with the global vouchers function hasn't been the greatest. The major sticking point for me is that (as far as I've been able to find) it does not support partial authorization and will simply say "insufficient funds" if the purchase amount exceeds the balance on the card. Instead, you have to look up the balance on the card (ctrl+shift+F3, check balance, scan the card), back out of the balance check window, enter the tender window, manually type in the current balance in the gift card space, and then type the amount of money paid in an alternate tender into the appropriate blank.
If your current solution has more functionality than this, it may be worth it to just stick with what you have.