If I understand you post correctly, you already have a 3-point hitch hydraulic post pounder and would like to use it on your 863 correct? If so, SkidSteerSolutions makes a 3-point adapter for skidsteers that looks like it would do what you like. I am looking at the EZ-PTO from them, which is the 3-point hitch with a hydraulic PTO motor that could power PTO driven tractor attachments. If you have any other PTO driven implements, this might be the way to go. Hope this helps.
I also have some fence posts to drive in this summer. They are 7' long metal t-posts and a hydraulic post pounder sounds great. Are you driving t-posts or wooden posts?