I always cite the best part of boardgames is the ability to house rule and fudge anything you want. That can still work here, but you would have to do some self-regulated house keeping to keep it balanced. Maybe you have a friend dropping in for one mission, or want to role-play having picked up an extra set of rebel hands.
I say go for it, just then you need to track xp card upgrades for that character (honestly doesn't seem hard)and juggle the activations (still seems easy since the game gives you 4 activations no matter the hero count).
The only quirks you might have is that an enemy activation will never explicitly target that character which can be both a pro and a con as it means other heroes with less health get hit more, but that hero will reliably be less targeted. You could mitigate this by rolling a dice every time the game targets a named hero to see if they should target your added hero(es) instead. Maybe on a 1 or 2 of a d6 if adding a third, or 1-2 on a d8 if adding a fourth.
The other quirks that might pop up is the equipment vendor seems to emphasize gear oriented to whomever it knows is in the party.That may mean your added heroes use gear that is sub-optimal for them.