I am buying a Sram Force 1 groupset with a new bike and I cannot test it before buying. In the case I really dislike the gearing steps issues, how difficult/costly would it be to convert to Sram Force 22 which has two chainrings?
Obviously you will be looking at a new crank, front derailleur and left hand shifter; and possibly a cassette.
I think you'll be OK running the Force 1 rear derailleur with double chainrings, unless you are getting a long cage version, in which case the minimum cassette size is 11-36 (source). If you want a narrower cassette - which your question implies - you'd want a mid or short cage that support 11-25 and 11-28.
You are looking at considerable cost - half a groupset. Cranks are easy to swap, left shifter not too hard - but you'll need a brake bleed for the front brake which is a pain. Front derailleur is not too bad (make sure the bike comes with front derailleur mounting options!) It does all add up to a significant effort though.
I see questions on this site about buying a bike then swapping out some big chunk of it from time to time. My advice is buy the bike that you want in the first place!. I'd at least try to get a test ride on a SRAM 1x bike before committing.