Manually, select the note-column, cut it out (alt-F3), in the Advanced edit, check the “Mix-Paste” checkbox then alt-F5 to paste the contents into the track where you want to merge the content.
I thought someone wrote a tool doing something similar, but i can’t find it.
The Piano Roll Editor tool has a sub tool that does that. It actually does two things. Separate a column into several columns the notes of the 10 octaves. Or merge all the notes into a single column, where possible.