If you have clients bringing in vouchers which were issued prior to you using Simple Inked, you have two ways you can use these vouchers to pay for the clients sale in POS.
1. For all sales being paid for using a voucher which was issued prior to Simple Inked, simply process the sale through POS using the 'Gift Voucher With No ID' payment method.
2. Alternatively, re-create the Gift Voucher via Admin > Vouchers & Packages, and sell it to the client via POS (this process generates a Gift Voucher ID) ?add client to sale, add Gift Voucher to sale, 'Receive Payment', choose payment type which was used when it was previous sold, 'Finish Sale', write VOUCHER ID on the voucher. Then add the client back to POS along with the items they are paying for, 'Receive Payment', select 'Gift Voucher' as payment type and enter in VOUCHER ID, 'Finish Sale'.
If you do it the second way, you'll need to backdate the first transaction (which sold the voucher to the client). To do this 'Edit' the transaction via the 'Transaction Report', change the transaction date, and 'Save'. Change the date to the original day when the voucher was originally sold to the client.
Obviously the second way is a little more convoluted.