Looks like you have hit a milestone (I just checked their app store…)
There is no easy way to manually add an extension. Best is to do what you did: ask the developer if he wants to include it in the extension repository. But If I understood it correctly, this extension only runs on the Athom device and this device already has a distribution channel via its own app store. @MvM, please correct me if I’m wrong.
The other way around may make more sense, i.e. have the extension manager available as an app in the Homey store in order to run other Roon extensions on the device. (Don’t know how feasible this is, just thinking out loud)