You can use the "Guess values" (import tag field) action for the field:
%dummy%(%composer% %dummy%)
THis will probably work only for 2-word-strings inside the parenthesis.
Fine if it worked.
You still could have used the action of the type "Guess value" for COMPOSER and the pattern
%dummy% (%composer %dummy%)
or
%dummy% (%composer Remix)
It would have been nice to see the regular expression that you used for your solution.