This is not possible, by definition. The Base58 string is the hash of the raw public key, and all hash functions are designed to be impossible [infeasible] to reverse. This is why they are commonly referred to as "trap-door functions."
If you are going to create a lockbox, you will need to get the full public key from the wallet. If you have an address string, it has already been hashed, but the wallet always maintains the full public key. You can right-click on an address in your wallet properties, or simply use the address book within the "Create Lockbox" window to fetch the public key from one of your loaded wallets.
Thanks so much for the quick response, that makes a lot of sense. Really enjoying testing out this feature.