It is my understanding that once you sign a particular section, that you cannot change it without removing the signature first. Thus, since you have a repeating section (that has been signed), adding an optional section inside that repeating section would invalidate the signature on that repeating section. That's why you are not allowed to do it.
If each manager needs to sign a section and do updates before their signature (i.e. add sections, etc), they will all need to be in separate sections and the signings will need to occur on those separate sections.... and not sign the outer section.
It will likely involve some thought and redesign to make it work for you, but I'm sure you'll be able to get it to work.