A couple of things............

If the bike has a freehub, yes, it's crazy easy. If it has verticals, you will need a tensioner. If that's the case, unfortunately you will have to buy the kit (or kind of.....see below for spacer ideas).
If the bike has a freehub and has horizontals, you can get away with almost no money for the conversion. You won't need a tensioner with horizontals, instead of putting cash into spacers, you can use a scrap piece of 1 1/4" PVC* to space your rear splined cog fore and aft for the proper chainline and use a lockring from an old spent freehub cassette.
If there was a 7 speed on your rear wheel initially, there's a chance it could be either a freehub or a freewheel cassette. If it has a freewheel cassette, you cannot just pull it and spin on a SS freewheel. Your chainline will be off considerably.** You will have to respace and redish the wheel for sures. In any case, we'll accomplish anything we put our melons to. As the great Philosopher Bob the Builder always says.............."Yes we can!"




*i paint mine black
**painfully so***
***quite literal i assure you