Retera Model Studio was made by rewriting huge portions of the code in the Matrix Eater and adding on top of that to make a better program that subsumed most of the same features.
So whether you refuse to upgrade to Retera Model Studio and just stay on Matrix Eater or not, both of these programs have "File ->Import" which puts the contents of one file into another file.
Here is a YouTube video where I use the feature to try to explain it and make it make more sense:
That feature is totally overkill for putting illidan's horns on kiljaeden, though. On Retera Model Studio you can probably just select the horns, use CTRL+C to copy them, open up kiljaeden, use CTRL+V and paste them, then select them and use
Re assign matrix
to link the animation of the horns to the kiljaeden head, and call it a day.
If your computer is Windows XP old timer stuff and Retera Model Studio doesn't work on there anymore but Matrix Eater does, you have a couple of options. Matrix Eater uses java 8 and there were historically versions of Retera Model Studio written on Java 8 so you could go and find a backup of that. Or, if that didn't work, you could go and find a backup of
Oinkerwinkles Geoset Merger
which is how I would put horns on kiljaeden when I was a kid in 2006. The
Oinkerwinkles Geoset Merger
was a tiny simple program, probably written in visual basic, to do the one specific task you are trying to do.
So basically it would show a small window, and you choose input file, and you choose "geoset file" (the horns), and then you choose output file. Then you can pick in a dropdown what bone the "geoset file" animates with (in this case, Bone_Head of kiljaeden). Then push "Merge" and it gives a popup saying "Done" and then you have your completed model.
That's how it worked when I was learning to do this stuff but I ended up switching from those specific case tools like that to a more general tool that I wrote myself, Matrix Eater, which was later replaced by Retera Model Studio