As far as I am aware, if you have purchased your key directly and convert the key to Steam you can still use that key.
I've converted one of my other keys to Steam and still run just fine using the regular launcher.
But as for the add-ons and stuff, you can just copy it to the Steam's Blockland directory.
If you're running a mac, there is a problem of preferences not saving, I believe it's a problem with Macs not giving Blockland the proper permission to write in the library directory where Steam installs Blockland.