What exactly do you want to customize for the services?
Let's say I want to customize the name of a specific service before it passed to the Dynatrace Server
You can't change the service name with a plugin, but you can define custom service names in the settings. Here is the Documentation. The service names are highly configurable that way.
What would be the use case to do it with a plugin, and not in the settings?
What if I want to customize the metrics of a detected service by Dynatrace?
Again, what would would be a possible change you would like to make?
If you are missing Metrics you can add additional ones per Plugin or API, depending on the use case.
I saw in the OneAgent SDK documentation where they talk only about customizing process using a plugin. That's why I wonder about the possibility of customizing a service through a plugin.
Yes if the default settings/capabilities are not enough for you, you can always extend the functionallity with the SDK. But the first step should always be to try it with the default OneAgent capabilities.
Oneagent SDK does not have such feature at the moment. Maybe @Jakub M. will share if is this on the roadmap. Currently you can use Oneagent SDK for sending metrics just for process groups and hosts and do alerting on those metrics.