changing only part of a tag


I was wondering if it is possible to change only part of a tag but not the whole tag.
BY this I mean the following:
I have a list of files where by each song tile is a different Artist. Since it comes from a compilation I do not want to put various for "artist" but I would like to place before each file artist name something like 'BB". SO I would get something that looks like te following:

BB This Singer
BB That singer
BB This DJ
BB That DJ

NOte that 'BB" is what i would like to add to all the files.Up to now the only way I know to do this is by doing so one by one-a very long process. Is there a way to automate this?

Thank you



I'm not that tag litearate. The one thing that seems to apply to me is:

I did that and replaced "xyz" with 'BB' and I got the BB in my comments field. In having understood this I then changed the COMMENT field to Artist and it replaced what was already there ( an irreverceable action BTW) In my case with the letters BB. which is not what I want. One does not need to create a new action to do this. SO obviously I either misunderstood what i read or did something wrong somwhere.
or perhaps I did not exaplin myself well. So I will try again
In one action/command I have files named:

This Singer
That singer
This DJ
That DJ

and I want to change them to (Including the space after BB):

BB This Singer
BB That singer
BB This DJ
BB That DJ

So that the title remains but the letters BB and the space are added before each of these.
What do I do?

Thanks again

Action type: Format value
Format string: BB %artist%

I think this is derivable from the comment example.