I’ve jumped FEI fences that were close to the HT course. One instance was back in 2006? at Poplar Place; I was running prelim on my intermediate mare who was always a bit suspicious of water. The CIC* had an element at the first water that was directly in line with my prelim fence; you were supposed to curve around it, but it was also sensible to jump the two of them together. (I think both of them may have been flagged A,B for the CIC course). I planned to do the extra CIC fence, and knew it was within the rules, but I approached the TD before xc just to make him aware of my plan. It was Roger Haller. I introduced myself, politely explained that I intended to jump an extra fence, and asked him if that was ok. He was really snarky, and said I should go read my rulebook if I didn’t know if it was legal. (I knew it was, I was just trying to be respectful!)
You are allowed to jump anything flagged, in the proper direction, provided you can do it safely. I will certainly “go larking” (hunting term!) if it makes sense for whatever horse I’m on. It’s pretty rare with modern course design of portables and new courses every weekend, but back in the day when fixtures were more permanent, and schooling days may not have been available, you might try something on course in preparation for a move up.