iTunes credit

How can I transfer credit to a redeemed gift card

That is not possible, or do you mean: from a redeemed gift card?

If so, that is also not possible.

When redeeming a gift card, the amount will be credited to your Apple Account balance, see -> Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)

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If you have an Apple Account balance that you can’t use

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•               Your Apple ID.

•               The refund amount.

•               Your current delivery address.

If your Apple Account balance is less than the cost of a single item, and you want to clear your Apple Account balance, -> contact Apple Support directly, they will help you clear your Apple Account balance, click -> Get Support

If you then want to change Country or Region, you must comply with all regulations described here -> Change your Apple ID country or region -> Change your Apple ID country or region – Apple Support (UK)

Click here for more support and assistance.