I don't think this is possible on a decentralised chain. Your private key (or seed words) has to be unique across the network. The auto generating algorithm used is a deterministic random one. Meaning someone else will not accidentally get your private key. If a network allows user generated private key, then there has to be a central place where you "submit" your private key to, and then this central place checks if your key is unique or not. (not sure but may be possible with masternodes?)

Also this should not be allowed in any case, because you know what? users are stupid People will try to generate seeds called "seed" or "secret password 123"

In any case it is too much buck for very little to no bang