How do I categorize the donors in a downloaded deposit that was transferred to our checking account as undeposited funds and which has already been reconciled?

Well, if it is a deposit of a Payment received from a customer, a Payment that paid an invoice from that customer (donor) then there's really no way to do that and no reason to.

However, if it was not a deposit of a Payment, then edit it and change the account, and then add as many rows as you need to break out the deposit as appropriate, putting a donor on each row.

There shouldn't be any reconciliation issue when doing this, as long as the total deposit stays the same.