I haven’t tried this in the direction external app->Mylio as Mylio does an excellent work in face recognition.
The other way works pretty well, in my case Mylio->IMatch.
At least in my case with the raw+jpg pairs.
With jpg only you have to write the data to jpg files manually (Photo - Save metadata to file). I don’t know what happens if you change data in multiple software simultaneously.
Mylio reads the file if it recognizes a change outside and updates the database.
But what IMatch does if its database has different data than the newly changed file from Mylio I cannot answer.
In Mylio you can invoke face tagging from multiple view.
If you have many new faces, People view - Untagged faces Batch processing is your friend.
If only a few, you can go picture by picture.
Batch processing is started when you are viewing multiple photos in a grid.
If you want to add a face not recognized by Mylio just click on the photo and hold the mouse until a selector field appers. You cannot change the size of the box, but otherwise you can add anybody.
If ín batch mode something is not clear, I doubleclick the photo and assign the faces there.
Under the … menu there is also an option “zoom to face”, but I have it at default.