It is not something that can be done without using some Visual Basic (VBA)
Code

If you have each of the individual documents in a separate section in the
assembled document, with the page numbers of each section formatted to start
at 1, and after assembling the document, you run the following macro:

Dim i As Long
With ActiveDocument
   For i = 1 To .Sections.Count - 1
       .Variables("Section" & i & "pages").Value =
.Sections(i).Range.Information(wdActiveEndPageNumber)
   Next i
End With

And starting with Section 2, you have the following field construction where
you want the page number to appear

{ = { DOCVARIABLE Section1Pages } + { PAGE } }

where you change the 1 to 2 in Section 3 and so on, then, we the fields in
the document are updated by doing  a print preview or printing the document,
the page numbers should be continuous throught the document