Is it possible to put a dweller on coffee break?

No, not really. Coffee Break is the status for an unassigned settler, from my knowledge this only happens in the following ways:

• Transitioning from Child to Adult
• Swapping work assignments with another dweller who is on coffee break
• Returning from the wastelands
• Being replaced with someone of a higher skill in the current room the dweller occupies (When the room is at capacity).

With this in mind, if you wanted to force a dweller on a coffee break the easiest way to do so without changing any other assignments would be to send the dweller to the wastelands for a few seconds and then recall them.

YES. Send them out to wasteland and recall them right away. They will be on coffee break. Cheers!