You can manually put something in the comments area or in positions 1-5 of the source. Something like b001, b002 … e002, e001.
Anything you do manually cannot be guarenteed accurate.
The best solution is to use Free Form RPG and indent your code.
If you have a large program and it is not Free form, another option if you are using SEU, is to use the X option on the lines. If you have not seen this before, put the cursor in the sequence number column at the left of the screen and use F1