Workbench mechanical is just another way to create and post process models (so instead doing that in the MAPDL UI/enviroment).
In the end of the day Mechcanical will create a ds.dat (MAPDL) file which MAPDL solver recieves and solves (that is why above MAPDL UI just reads the ds.dat file created in Mechanical a shown in the image).
That is just some background info.
Now to your question, and for Mechanical to recieve an APDL file that is also possible. You would need to save the MAPDL .db file as .cdb file (use CDWRITE or from UI Archive model).
The cdb file can be read into workbench via the External model system (see below):
See here for more info and a tutorial of what I explained above: