It took me entirely too long to find instructions on exactly how to do this, so I'm writing a concise guide here.

Scenario:

You have a non-HD wallet, and you want to convert to an HD wallet. Or, you have a specific receiving address, and you want to give all the coins received by that address to another person. You want to transfer these coins without a transaction fee.

Requirements

You need to have a non-pruned full node in order to do this.

How to:

1. Load up the wallet.dat file(s) which contains the coins and address(es) which you need to transfer to the new wallet.
2. Go into receiving addresses and copy all of the addresses which have recieved coins in this wallet into a notepad or similar tool.
3. Open the debug console
4. If the wallet is encrypted, type in walletpassword "<your wallet password>" 600 this will give you 10 minutes to fiddle with addresses before having to unlock the wallet again. You can type walletlock to re-lock the wallet at any time. Do not include the <> in your password.
5. dumpprivkey "<receiving address>" Repeat this command for each receiving address you have. Do not include the <>. Copy-paste the private key into the notepad beneath the key you got it from, it will be longer than the receiving address.
6. Close down your wallet.
7. Swap your wallet files around such that the new wallet (the one you want the coins in) is wallet.dat
8. Start up the wallet
9. Open the debug console
10. If the wallet is encrypted, you'll need to use walletpassword.
11. importprivkey "<priv key>" "<recieving_addr_label>" false If you are entering more than one private key, use the 'false' keyword at the end. However, remove 'false' for the last key you are adding.
12. Wait 10-30 minutes while your client rescans the entire blockchain, single-threaded (FOR SOME REASON!?! OMG SOMEONE MAKE THIS SHIT PARALLEL!), for transactions that used those keys.
13. When the rescan finishes, you should see all the coins received by that address in your wallet.