If you're viewing the CSP using 'Dynamic Rung Display' on a GE Mark V HMI (running MS-Windows and CIMPLICITY) I believe the Dynamic Rung Display automatically opens sequencing BBLs to see sub-rungs. (One can't see "inside" other types of BBLs; only the inputs and the outputs.)
For GE Mark V HMIs, there is a description of the Dynamic Rung Display in the 'HMI for Speedtronic Turbine Control Operator's Guide', GEH-6126, in Chapter 3.
If you are using an <I>, there should be a button on the screen in the BBL block called SRC which allows you to "open" a sequencing BBL to see the sub rungs.
For <I>s, there is a description of Dynamic Rung Displays in the 'Speeedtroinc Mark V Turbine Control Maintenance Manual', GEH-5980, Chapter 4, Section 2 (4-2).
You cannot add a sub-rung to the sequence code of the MkV. Subrungs are developed in Salem and become part of the landscape, selectable based on items such as type of cranking means and the likes.
This said, Yes, it is possible to modify a sub-rung provided you understand where they reside and the language needed to make the modifications along with the limitations of keeping the change in place once completed.
