I´ve
researched asked Meridian about adding custom stats, its possible but they can't be seen in stats screen (since it can't scroll).
So basically there could be e.g. a "rifle expertise" stat (initial 0), that increases on kills with rifles, by commendations. If that stat is set in bonus formula (like acc + rifle expertise) you could have weapon expert levels.
Even for unarmed combat
Only problem is you can't see them in stats, so its not transparent how strong your soldier actually is with current weapon.
But there might be a way, there´s a mod that displays some stats in inventory screen (its really helpful when equipping), so maybe those stats could be displayed there too. Or somewhere else in short form.