Oh how I wish I could do that... easily at least.
I have been able to remove something from a group using Pro/Program, but it's a very careful process, especially if it is the last feature in the group. You need to go into the Pro/Program, cut out the bits that point to your feature, and make sure that the group's definition no longer refers to that feature, too. If it is the last feaute, one of the group tags will refer to the last feature in the group and it's feature ID. All of this has to be replaced with the new last feature.
Adding something in would likely be a similar process. You should create the feature, then try to rework the pro/program so that it falls within the group. Before doing this, bring the number of pattern instances down to one (pattern feature might not let you do this, but I've found that making a parameter and then setting this as a relation sometimes tricks Pro/E into a one instance pattern).
Good luck