I am finishing off my attic space. I have completed all the framing but realized after the fact that I did not think about putting 1/2 inch plywood between my 2x4 headers on the partition wall door openings. I need that extra 1/2 inch to attach the trim to. Is there anything wrong with attaching 1/2 ply to the outside of the 2x4 headers. My headers are 2 - 2x4 on edge. I alligned them with the outside opening. i am proposing putting 1/2 inch ply on the inside to bring the header flush with the inside cripple studs. I should have thought about this before but I am new to this stuff. I am being inspected next week. Any help would be appreciated.

No problem at all with putting the 1/2" plywood on the outside of the "headers". Now, what may be a problem is if these are actual headers that are carrying a load. If so, double 2x6 is the minimum that will likely get passed by the code guys.

2x4 will work for a non-bearing door frame, not for headers carrying a load.

These are interior partitions with no load. Thank you for the reply.