"How can I make passive item bonuses stack in Dota 2?

[rigid hemlock]

Do I need to change something in addon_game_mode, or would it be better to make it an ability?"

[marble niche]

Your question is a little vague, I can interperate 3 scenarios from it

1. Intrinsic modifiers already stack, make sure you're actually applying them correctly
2. Modifiers added by the intrinsic modifier (not the intrinsic modifier itself) need the MULTIPLE attribute so they can be applied multiple times
3. Items with stacks (charges) need to be specifically handled to alter the value given by the modifier based on the charge count (ie; deso ad stacking)

[rigid hemlock]

for sange and yasha for example

it cannot stack i think

or basher

like, is there a way of stacking it by an ability or addon game mode?

[marble niche]

Remake it so it does

Its not a toggle, its a hard coded behaviour

[rigid hemlock]

ohhh i understand now