Maybe you can take a look at this thread where something similar is discussed:
In a nutshell: You have to override the functions prepareListObject or afterRead of the class modRestController.
To simplify the code, it would also help, if you defined the relationship between the two tables in your schema.
btw: If this is your complete schema, then you also have to define the fields createdby and editedby or the aggregate-relationships to modUser won’t work.