Transferring an iTunes credit to an App Store credit

if it's possible to transfer an iTunes credit to an App Store credit?

What do you mean?

An iTunes credit and an App Store credit are the same, in the sense that both are in your Apple Account balance.

See -> Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)

I received a voucher which when I redeemed automatically credited my iTunes account

By that voucher you mean an Apple Gift Card or App Store & iTunes gift card?

Yes, if you already use your Apple Account balance, the Apple Gift Card or App Store & iTunes gift card is automatically added to your Apple Account balance.

See also -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

• Find out what you can buy with your Apple Gift Card or Apple Account balance.

Check also ⤵️

Apple Gift Card and Apple Account balance can't be used for some purchases

• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.

Learn more

• To add funds to your Apple Account balance, use the payment method that you have on file.
• Find out how to check your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

Some stores and apps might not be available in your country or region. Find out what's available in your country or region.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)