As per the QR website FAQ page. “The travel voucher may be used to purchase a Qatar Airways ticket (ticket number should start with 157), with or without another airline in the journey.” Though I’ve never personally tried this before.
I also had a question regarding the vouchers. If you use a voucher and have residual value remaining are you able to use the same voucher for another booking or do they send you a new one? Is the process quick/easy?