Having done this very thing last night - I always swore I wouldn't, because I resent having given Frontier a small fortune for cosmetics then not having access to them when I'm playing the game, but that £10 Steam offer was just too tempting - I ought to be in a position to advise you, but I can't.
I made sure my bindings were backed up (I backed up my journal files for good measure). I set up a Steam account with a variant of my Gmail address (put a dot somewhere in your name, Frontier thinks it's a new address but Gmail ignores the dot). Steam insisted I download a new copy of the game. When I tried to run it the launcher popped up and I opted to create a new Frontier account using the variant email address. Then when I tried to log into the game I was asked to enter an authentication code Frontier would email to me. I got the code and entered it.
The launcher refused to recognise it. I tried again. About fifteen times. What's the definition of insanity again?
Each time the launcher either refused to recognise the code or told me I didn't own a copy of the game. Insane as I am, eventually even I gave up.
I then fired up my original copy of the game to see if that still worked. Not unexpectedly, my bindings were gone (successfully replaced) but I was where I should be. I haven't checked the journal files yet, though.
I then decided to have one last hack at the Steam version. I clicked Run on Steam, Steam asked me if I wanted it in VR (I did), and lo! the game just loaded up with my fresh commander - no sign of the launcher, it just loaded. I have no idea why.
At this point it was three o'clock in the morning so I went to bed. Clearly at some point something went right but after less sleep than I like I'm up and have absolutely no clue what or when.
A message to Frontier: in the name of the Full Spectrum Scanner, give us multiple Commander slots. Or sell them to us, even. I don't mind paying; I just don't think you should be putting your customers through the waste (I now have two copies of the game taking up precious SSD space) and misery of setting up a second account. Oh, and access to the cosmetics I've paid you a fortune for would be nice too.