since scribus has multi pages documents, it's not common to merge layers...
if you have only few items to move around, the simplest way would be to cut and paste the items, page per page.
it should also be possible to create a python script that moves all items from one layer to a different one...
i guess that it should not be that hard either to ad it as a scribus action... but i've not heard about anybody asking for it in so many years... so i guess that other features have more priority...