Extended Tag Contengroup Problem

I am working on a friends music and have a problem. The songs have been Tagged using the CONTENTGROUP field. The field is being used to help create playlists. The problem I am having is that I want to add additional text to the CONTENTGROUP field while selecting multiple songs. When multiple songs are selected the default is < keep > for all the extended fields including CONTENTGROUP. Is it possible to keep the existing text while adding new text when multiple songs are selected? Here is an example...

Existing value - WPDA, WCXR
Modified value - WPDA, WCXR, WSUN

I have tried to modify the value like this < keep >, WSUN but this does not work. It simple changes the field value to < keep >, WSUN. I want this to be the result WPDA, WCXR, WSUN.

Does anyone have an idea of how to accomplish this? I think it would be useful for many other tag fields as well. If not would the developer consider making a change to solve this problem?

Thanks for the support...

You would have to create an action of the type "Format value" for the field CONTENTGROUP
Enter as format string
%contentgroup%, WSUN
if you want to add the string WSUN to the current content of CONTENTGROUP.
An action works on all selecte files.

You would have to modify the action (or duplicate it) for every string you want to append.
Alternatively, you could add a user-defined field like MY_CONTENTGROUP and fill it with only just but all the strings you want to add.
To add the contents of MY_CONTENTGROUP to CONTENTGROUP use the same type of action as described above but modify the format string as follows:

%contentgroup%, %my_contentgroup%

Thank you ohrenkino. That is a great solution...