I have an old Steam SK account (Back from the Mist days) that I have left alone for a while (like 2 years). I recently added Steam to my new computer but have not installed SK on Steam. I am wondering if I install SK on Steam I would be unable to access my current non-steam account. I am also a bit fearful of having the Steam SK account overwrite my current account.
I would like to go on my SK steam account and poke around a bit, maybe end up changing my current account name to my Steam account name (after transferring what I earned on Steam to my current account and then deleting it or something).
I don't understand exactly what your concern is, but steam allows you to both log in with steam accounts and normal Spiral Knights accounts, It does ask you if you want to transfer your stuff to the steam account if you're playing using steam but I am not sure how that works since I've never used it myself, actually "i've never used it on an account that wasn't empty" would be the correct way to say it.
Also you should be able to install both Steam Spiral Knights and normal version in the same computer if you want to log in with your non-steam accounts without actually having to use Steam.