Just checking the 9.1 docs for implementing deployer extension via Add-on:
Unlike other Extensions, the Content Deployer does not allow for multiple Extensions to be added to it at the same time. Rather, when you installed Content Deployer, you added an Add-on containing one single Extension, possibly containing resources for multiple Content Deployer enhancements (say, both for Experience Optimization and for Context Expressions), and to add your custom Content Deployer enhancement, you'd need to add resources and Pipeline elements to the existing Extension.
So if you've already deployed an Deployer extension via Add-on, say the Context Expression Extension - Deployer DX Extension, you'd have to modify this SDL add-on to include your own custom extension and then deploy just this combined extension?
You can't deploy your own custom deployer extension as its own Add-on?