Hi,
You will not be able to redefine a group and add new features to it, you have to delete un group and create a new group to include in the new feature. If you have a pattern of a group and want a new feature to be part of the pattern, then you can create the new feature and then pattern using the reference option. this would create a pattern of the new feature over the existing group pattern. This would help if you are not going to delete the existing features.