No, you cannot achieve that directly with current extensions. They operate only on a custom device group and custom devices level only.
However, services on top of these custom devices are created from opaque traffic detected by OneAgents. That means if a process monitored by OneAgent communicates (HTTP call / DB service) to a custom device (matching done by IP addresses + ports) a service is created on custom devices. So the source of the service on top of custom devices is in fact the opaque service.
It's a really good point made by Julius about IP address and ports. It's not mandatory to create a custom device with such detail in Dynatrace, so it's often overlooked. Should be considered best practice to ALWAYS create a custom device with valid IP address, port, (and DNS names) - even outside of ActiveGate extensions, this can be used to split out 'unmonitored' requests to separate entities in Smartscape. This is the most straightforward approach for independent service-level tracking where a full-stack OneAgent is not deployed to the component.