Your best shot would be to do something similar to EW's Siege The Day
mod. To do this, starting from a civilian unit, you'd need to:
- assign your civilian unit its desired strength in the RangedCombat column
- create a promotion that gives a RangeChange of your unit's desired range
- assign the promotion to your unit in the Unit_FreePromotions table
I haven't tested this myself, so I can't guarantee that it works, but that's what I would try in your situation.