Harris radios can be programmed onto Moto (or other) P-25 Phase-1 systems without issue. No system key ever required, as Harris radios prior to RPM R12a did not even support the system key concept.
Part of what I did the last few years before I retired was to assist the Sales Team on demos of Harris equipment on non-Harris P-25 systems. I would quietly arrive a few days ahead of the scheduled demo date, passively (and discreetly !!!) work-out the required programming for the prospective customer’s system, and verify the data and programming by receive only operation.
Once the Sales Team arrived, and we had written permission to program radios onto the system, with valid radio LID’s and which talkgroups are authorized for the visit.
From that, I would update my previously reverse-engineered programming file, program the radio(s) with valid ID info, and hand them off to the customer and Sale folks.
Several years ago, there was produced an official document on how to program radios onto Moto systems. Done in a ‘cookbook’ format, to walk someone through the process, were the only information know on the target system is the licensed frequencies.