Hi,
It's straightforward to have a custom action use a single DLL that's an attached file, but I don't see a way to attach multiple files to a single custom action.
I'm afraid that this is not supported by Windows Installer. You can add all DLLs to your package and use a "New Installed Custom Action" to call the main DLL. Note that in this case you need to scheduled the custom action after "InstallFiles" standard action with "Deferred" as "Execution Options".
If you don't want to install the DLLs with your application then you can add those files as "Temporary Files":
http://www.advancedinstaller.com/user-g ... files.html
Best Regards,
Gigi