Terminology is very different in NGS.
So what happens now is that you
(1) check whether original releases releases and tracklists are in the DB. If not, add them the normal way you would. Forget about the box set for now.
(2) Add a new RG and Release for the Box Set with the name of the Box Set, release date/label of box set etc - you can do this at the same time, you don't have to add the RG separately
(3) Now you use "Add disc" with "existing tracklist" to search for the tracklists from the 5 original releases already in the DB.
So what you end up with (2 discs for simplicity)
RG Album 1 > Release Album 1 > Tracklist A > N tracks > N recordings
RG Album 2 > Release Album 2 > Tracklist B > M tracks > M recordings
RG Box Set 1 > Release Box Set 1 > Tracklist A > N tracks > N recordings
Tracklist B > M tracks > M recordings
So the Box set now shares the /tracklists/ with the original releases since they are identical. Think of a release in NGS like a "release event" was in old-MB. It's really just a set of labels/cats and a release date/country.