During the build process, VIs are stripped from features that are not used at run time. They loses their icon, diagram and front panel (for subVIs). As such, you can't modify them anymore.
However, it is possible to extract them from an executable by renaming a copy of the file with a LLB extension. You can browse the LLB to put a VI within on the diagram of another VI. You can even open the Top Level VI and run it in the development system again and see the VI hierarchy.
Ultimately, if you want to change the functionality of one compiled VI, create a new VI with the same name and connector pane and save it over the previous VI in the LLB (the VI or its callers must not be in memory).
