Right-click on the collection, select Export This Collection As Catalog, uncheck "Export Negative Files".
Then in the main catalog, select File->Import from Another Catalog and point to the catalog you created based on the instructions in the previous paragraph.
By the way, thijs is in effect a "Copy" and not a "Move" of this collection.
Of course, this will also bring in the adjustments and metadata applied to those photos, updating the main catalogue. That may or may not be desirable.
it does NOT work. I don't know why!
Of course when I import the collection's catalogue I deselect the option to import the raw files as they are all already in place and metadata are already updated (as postproduction is the correct one). LR imports the catalog but nothing new appear under the collection's list ...