Yes! To use a Voucher/Open ticket please visit the website of the travel provider that issued the voucher/open ticket. Select the flights you want, enter the passenger details, and on the final payment screen, you'll find an option to use credit/voucher.

You have to add the old PNR NUMBER in the relevant section on the final payment screen and you'll be told how much credit you have. You'll also see if anything else is required to pay for the new flights.