How to find/construct simple trim for windows
Yes you could add another jamb extension to the jamb extensions that are there already. The down side of that is the joint will look out of place if you try to add the new wood flush with the original, or if you step it back to make a new reveal, (the reveal you referred to is more of a projection of the wall or recess of the jamb extension) to me it makes the inside of the window opening too &quot;busy&quot;.
If it were mine to trim I would use 5/4 material (1&quot; thick&quot;) and table saw a relief to accommodate the wall projection.
That being said, if you have trim elsewhere in the room that you will need to match up and it is not thick enough to cut the back out, then you may need to add to the jamb extension, so you can use an off the shelf trim.