Monitor does not properly import extensions at 10.6. Or, maybe more accurately, the folder structure of extensions offered from the Gallery is not properly formatted.
In any case, after importing, Monitor reported the the import was successful. However the type dropdown in a New Task setup never included my extensions, even though their respective folders existed in the bin folder (..ArcGIS Monitor\Administrator\resources\app\bin).
I navigated to the bin folder and pulled the extension folder out of the root. Monitor would then recognize the extension.
Solved! Go to Solution.