Dear Kristian,
In Neptus, a Maneuver is a class file deriving from (subclass of) pt.lsts.neptus.mp.Maneuver. This class should implement the interfaces IMCSerialization (serialization as IMC message) and optionally LocatedManeuver (world location attribute).
On top of these interfaces, your class must be able to store and load its parameters from XML. Most of the times it is much simpler to extend some maneuver that already exists (like GotoManeuver).
Afterwards, you should include the XML configuration with the default values to the vehicle's configuration file that will have this maneuver.
A good reference may be looking at a commit that basically adds a new maneuver to Neptus:
Best regards,
José