So basically you want to put a timer on a transition but you do not want to set the duedate in the process definition but during runtime, right?
AFAIK there is no API to create jobs. ManagementService just exposes a method to execute jobs and to search for jobs by creating a JobQuery. If this is really the case I would consider you to fill a JIRA issue and to propose the extension of the API.
All jBPM object are in fact Hibernate objects. So you can simply programmatically create a Job with the correct date and save it through the hibernate session.
Best approach to this is to write a custom Command and invoke it through the CommandService (there are plenty such examples in the source code).