Thanks for the help so far. Ideally it should work like this:
On a host are x instances of an application running and not all of them should be monitored. Which one of these should be monitored is determined by another application that saves this info in a database. The checkmk plugin on the host gets this information plus the payload (the state info of the applications) and Checkmk fetches this info. My custom plugin running in Checkmk now creates separate services with labels, etc. that are discovered but I still have to activate them manually.
Ideally at this point I´d like to tell Checkmk in my custom plugin that there are new services to be monitored and that they should be activated automatically as soon as they are discovered.
@ChristianM I tried setting up an enforced service but this seems to be bound to one item/service. So discovery/activation of several instances of the same service didn´t work. But maybe I misunderstood.
@martin.schwarz I tried tweaking these discovery rules, but it seems I´d have to lower the interval down to 1 minute to be able to get the services as instantaneous as possible. Also I like to deactivate this option because it always creates a warning because of discovered but unmonitored services. The option to set it to OK in this case dowesn´t seem to work.