You can use INCLUDETEXT fields to include stuff conditionally. But I
wouldn't even go near normal Word "Master documents and subdocuments" to do
this - just use regular documents and ensure that any styles you use are set
up the same way in each document.

Then either use nested IF feilds with INCLUDETEXT, e.g.

Part 1 is the fist part of your mail merge main document. Then have
something like

{ IF { MERGEFIELD SecondPart } = 2
"{ INCLUDETEXT "c:\\pathname\\part2.doc" }"
"{ INCLUDETEXT "c:\\pathname\\part3.doc" }" }

Or, if you can relate the database field to the document name in a useful
way, you might use something like

{ INCLUDETEXT "c:\\pathname\\part{ MERGEFIELD SecondPart }.doc" }

All the {} need to be the special field code braces you can enter using
ctrl0F9

--
Peter Jamieson
MS Word MVP