You don't need to download the whole chain to do that.

You need the cli_wallet.

https://steemit.com/steemhelp/@felixxx/5-sbd-bounty-how-to-use-and-set-up-cliwallet-to-change-my-passwords

In the comment section of my post, you can find an easy method.

You need 4 keys to create an account. 

The command you are looking for is:  create_account_with_keys

create_account will create an account with the keys stored in your wallet.