The clean way is modifying the UE source, create custom installer and enforce everyone to use it.
The Epic workflow uses UnrealGameSync. So, I doubt that you can configure UAT to discover *.dll based on configure file.
Base on adding custom UAT project to *.uproject directory, It will eventually output *.dll to Engine’s Binaries Folder.
So, The hack way is copying *.dll to Engine’s Binaries folder.
Very likely that this will fail.
For more document about custom UAT which you may have already seen it, Gauntlet Automated Testing and Performance Metrics in UE4 - HŌRU