I am almost certain it is not possible to avoid some loss of features (pattern members) to get what you want. I concur with @KenFarley on what is required to satisfy your request. Without access to the model, it is hard to provide a definitive answer, but I don't recall ever being able to insert features into a group that has been patterned without deleting the pattern.
If you have the option to unpattern in the model, then you might be able to preserve the pattern members outside of a pattern but that would not seem to meet the requirements to add members to a group and include them in the existing pattern.