That idea is only really mentioned in the no-longer-canon novels, from what I remember. Realistically (I use that term loosely, in this case), a normal person wouldn't use a lightsaber because they wouldn't be able to use it defensively against a ranged opponent (you need Jedi reflexes to block shots), so they'd get eaten up the first time they come up against someone with a blaster at a distance.
In theory Finn (and all the stormtroopers) could have been (and should have been) trained in multiple forms of combat, so he could have been trained similarly to the guy with the blaster/electric tonfa hybrid with the presumption they'd eventually run into someone using a lightsaber, even if it wasn't another Jedi.