In your Field Mapping/conversion, you can code BREAK-POINT at the very first field. Then the program execution will break when it reaches there.

You can also put soft breakpoints in the generated code. To view your generated code, you need to bring it up in the initial screen of your object.

To do so, click on the User Menu button & check the Display Read Program & Display Conversion Program checkboxes. Hit ENTER. Now you will see 2 additional options on the initial screen of your Object. Double-clicking on the Display Read Program or Display Conversion Program will take you into the generated code. Place your soft breakpoint as you normally do.

Another option: go to the step DISPLAY READ PROGRAM or DISPLAY CONVERSION PROGRAM.
Now you will see the ABAP^TM editor with the code. Place the breakpoint (soft break point using STOP button on application tool bar) wherever you desire.