I'm making a custom feat that grants the player an additional d4 to their unarmed strike damage. For most classes/races, this would just mean the player gets 1d4 for damage. But monks already gets a d4 for unarmed strike damage, so this feat would grant them a total of 2d4. When a monk levels up with this feat, their unarmed strike will get 1d6 + 1d4 (and so on).This also applies to some races who get buffs to their unarmed strike damage.

The problem is I can't figure out how to edit the modifier in the homebrew editor to grant an addition d4 for players who already have buffs to their unarmed strike damage. I can set the damage to be exactly 1d4, but I can't make it give an additional d4 on top of their already existing buffs.

Is this possible to do? As of now, I'm just having my players roll the extra d4 manually, but it would be nice to have it contained within the action ability. Thanks in advance!

You cannot, the best you can do is to give it an Action to roll the additional d4 damage and the player will have to roll it separately in addition to their regular unarmed strike.