Are there any official rules on this

Yeah. There is no way to do it in the official rules.

As a GM I might say if you have the target paralyzed or unconscious it might work. Or perhaps controlled/influenced to friendly and you made a successful deception check they might willingly do it... but for the most part, you're not going to be able to force feed someone.

Tell me, if you're in heated combat, do you think you could be force feed something? No. Only if you were completely restrained (effectively paralyzed by the games terms) by multiple people possibly holding your nose as well to force the mouth to open then you could possibly try to get someone to drink, but then they could just spit it out.

Are your players being creative? Sure, but they're not thinking of a valid strategy. Perhaps if they want to do something like that they should look into poisons with the Injury Trait like Lethargy Poison. It really wouldn't make sense from a balance perspective to allow someone something that would be stronger than existing poisons that already exist in the game. Send them off in this direction to give them ideas.