the chance of equpment caching fire i think is not moddable, if you want fire arrows or even normal ones to actually damage the siege equipment you need to look into descr_projectile.txt file
Code:
projectile SelfBowArrow
effect arrows_new_set
end_effect arrow_impact_ground_set
end_man_effect man_impact_tiny_set
end_package_effect arrow_impact_wall_set
end_shatter_effect arrow_impact_ground_set
end_shatter_man_effect man_impact_tiny_set
end_shatter_package_effect arrow_broken_impact_wall_set
effect_offset -1.5
damage 0
radius 0.1
mass 0.05
accuracy_vs_units 0.05
affected_by_rain
min_angle -75
max_angle 65
velocity 45
display aimed
effect_only
i'd try chaning the "damage" to some number, 10, 40, idk
personally i disabled fire arrows so the suff i posted above is for normal arrows, generally you can easily distinguish between the two, the fire arrows are usually named "SelfBowArrow_Firery" or something like that
if it doesnt work then you can try looking at a balista/catapult projectile for reference and copy their stats so that arrows deal damage like them