Not entirely sure you could pull that off as a buff. Maybe as a new class that acted twice, and even then I would be skeptical. I would recommend checking out the enemies that have multiple actions and see how that works. There are also some enemies that say have 2 actions per round, but also have an auto-action that triggers at the end/beginning of each round. Check that out, see if it might work for you. I would recommend the Crimson Curse Croc as good enemy to look into. He implements all of the aforementioned mechanics.

DDBalance by wodzu_93 over on the Nexus has done this, or at least the documentation says it's been applied, so you might want to have a look at their code. I haven't actually played the mod so can't swear it works!

They've added a single extra action to the target of Emboldening Vapours when it's initially cast, which I think is an absolutely inspired solution to the action efficiency cost of single target buffs.

Update on this, turns out you can do this. It's an effect modifier called ".initiative_change X" where X is the number of total actions. Just found this out. Hope it's still relevant.

Cheers!