Here is a thread that deals with "featuring" - see if there is something for you:
You could split the field artist with "Import tag fields"
Source: %artist%
Target: %artist%, %myartist%
Then add %myartist% to TITLE with an action of the type "Format value" for TITLE
Format string: `%title% (feat. %myartist%)
then delete the field MYARTIST.
BUT beware: three are so many artists that have a comma as valid band names like
Earth, Wind & Fire
Portugal, the man
Emerson, Lake & Palmer
Bell, Book & Candle
I doubt that they should get split.