If you create an External Library - Copy the Group and Materials that you want to the External Library - then from there you can copy them into any group that you want in the project library.
There are some groups that are not compatible - like Earthen, Earthen Select and Landscaping but other than that you should be able to achieve what you want to achieve
I will log it in as a defect - it is not really that mission critical though and there may be good reason to not allow it - not sure what that could be - however there is a lot of cross referencing going on behind the scenes in Corridors and in Site Improvements and in Takeoff - so I can imagine that is why it is prohibited as changing it seems simple enough but then all the cross references need to be checked and validated - and it is in these areas that we tend to see exceptions later on because things that were referenced were deleted or moved etc - so likely not a trivial change
Alan