Custom properties for data source instance ?

Typically this is done via autodiscovery, but if you add manual instances you can manually define properties for the instances. For the AD method, each instance is generated with the normal fields followed by an optional list of property/value strings.  Assuming AD is run often enough, those strings should be current (more or less) for reference in custom alert messages via unconditional token substitution.

You can also use PropertySources to add auto properties if you want to do that without editing an existing datasource.

If you need examples, Arista_Sensor_Fans is one of many datasources that generates auto properties.  Or, look at almost any PropertySource module.

I would not add any manual properties to automatic instances as those would likely vanish at some point.