If the appropriate steps are taken, I wouldn’t see why not.
There is probably some legal reasons as to why they are not though. One of the only events not to be added to private Horde was the Terminator event on Training Grounds that was around launch. I don’t think it is a coincidence.
Plus, they were essentially DR-1’s with a T800 skin on them if I remember correctly? TC would have to go through some effort to make them not just a reskin of a Peacemaker or a DR-1. It wasn’t, well, exciting. It was cool to see the models but that was about it.
The Speaker would be cool, they can make them like the Gears 4 Scions, but again, I don’t really know how possible that would be seen as they don’t do it in Gears 5 at all, how much balancing would have to be done to get them workable.
But, we have Stim grenades, the HoD, and the Breechshot that are not being added because they would need a lot of work done for them to get into Horde, if TC isn’t willing to put the effort onto them, weapons that are already in VS, I cannot see how they would do this either.
If they were to be added I would expect the bare minimum, given TC have said that Drop 2 was essentially the last big update.
TL;DR: Would like them to be added but almost certainly will not.