iCloud Storage Upgrade Using Voucher

I have an iTunes account (which I have had for years) and an iCloud account and I want to keep them totally separate. The iTunes account works as it should do and when additional money is required I purchase and redeem a voucher. This allows me to use any purchases on my iPhone and iMac.

However, I only use my iCloud account on my iMac not on my iPhone. I now want to add additional iCloud storage. I know I can pay using credit card, Paypal etc. but do not want to do it this way. I only want to pay for it using a voucher. Is this possible and if so how do I do it?

Thank you