I was on the wrong path - instead of actually generating the final mailmerge I found a way to loop trough all 'documents'. While doing so the original fields stay available for manipulation withing VBA. Hopefully somebody else can use my trick to circumvent the not so productive mailmerge.
Find attached the solution to my problem:
Sub pdf()
'Splits up document per page
Dim numPages As Integer
Dim runner As Integer
Dim file As String
numPages = ActiveDocument.ComputeStatistics(wdStatisticPages)
runner = 1
Dim fileNameRunner
ActiveDocument.MailMerge.DataSource.ActiveRecord = wdFirstRecord
While runner <= numPages - 1
file = ActiveDocument.MailMerge.DataSource.DataFields("CODE").Value
ActiveDocument.ExportAsFixedFormat OutputFileName:=file _
, ExportFormat:=wdExportFormatPDF, OpenAfterExport:=False, OptimizeFor:= _
wdExportOptimizeForPrint, Range:=wdExportFromTo, From:=1, To:=1, Item:= _
wdExportDocumentContent, IncludeDocProps:=True, KeepIRM:=True, _
CreateBookmarks:=wdExportCreateNoBookmarks, DocStructureTags:=True, _
BitmapMissingFonts:=True, UseISO19005_1:=True
ChangeFileOpenDirectory "C:\output\"
runner = runner + 1
ActiveDocument.MailMerge.DataSource.ActiveRecord = wdNextRecord
Wend
End Sub