The short answer is you can't get it out of the box as debtor transactions are not exposed.
However ServiceStack does support auto query which means you can expose db_trans which is the table that you need to access, to see an example of how to do this look at any of the routes under queries, then see how that route can be added by having a look at how any of these routes are actually exposed in the REST API plugin, Jiwa have already created/defined the query and response objects in the Service Model name space so all you need to do is add the route and possibly service to do it.
You can also create a view and expose that but in that case you would also need to define the request and response classes also and then create the service.
Jiwa also have a sample plugin for defining additional routes that you could look at. The following link should also help
https://docs.jiwa.com.au/display/J7UG/A ... e+REST+API