So that I understand the issue clearly, let's use the example of a data merge, where a record contains a link to a 28pp INDD file that needs to place only page four. Other records may link to the same INDD file but need to import page 5, 7, 23 etc...
If I've understood the question correctly, I'm afraid that the answer is that this cannot be done using Data Merge by itself. That is, when the link is made in the txt/csv file for import into the data merge, it typically looks like this:
file.indd (assuming the indesign file is in the same directory as the txt/csv data file)
To my knowledge (unless any Adobe engineers are reading this and can correct me) there's no extra syntax that can be added to the link to denote what page of that InDesign file to import. This also applies to other multi-page items such as PDF or AI.
It's a shame too because it is something that I've seen many other users ask for. This would be a suggestion for Feature Request/Bug Report Form
Just wondering if you put in the syntax in excel to have it display something like
I've never tried it.