Call Merge-Modules custom action from Setup Project

Question

I do have a ddl project which implements some interfaces from other ddls. Those other ddls are 3rd party ddls. The 3rd party stuff provides merge modules (.msm files) for all their ddls. According to the 3rd party documentation, I need to call a custom action from their merge modules during the setup project. The 3rd party stuff is quite old and in their documentation they say:

Note - How to use this MergeModule in a Setup
- Import Merge Module 
- Call Merge Module custom action "RegisterIntopAssembly" after files are copied to target system
  (Installation\Execute sequence after the InstallFiles standard action)
- Set the condition to call "RegisterIntopAssembly" custom action to "Not Installed"
- Leave the MergeModule destination as "(Use merge module's default destination)

I have created a Setup Project, imported the msm Merge Modules but I do not know how to call the merge modules custom action in Visual Studio 2015 Setup Project. In the Custom Action view I can only add custom actions from dlls, I cant select there any merge module.

Any hints?