Another way to convert your custom string into an electrum wallet is to do a sha256sum md5sum of it. If you are on a nix system:
It's really really inadvisable to do this, short human generated strings have very low entropy even (or especially) when you think you're being clever about it. Many people have lost substantial amounts of Bitcoin this way.
Yeah well this is not about short strings. That was just an example. This is about converting user provided entropy into something usable. Ideally the input would be a string representing cards from a deck that the user shuffled. Or coin tosses.