Try an action of the type "Guess value" for ARTIST
Enter as guessing pattern:
%artist% feat %another tag%
This leaves out the "feat" but that should be easy to add to the %another tag% field with a format value action.
Just a personal note:
The tag is all the metadata. The tag can follow certain conventions and usually is described as V1 tag or OGG or FLAC tag.
A tag then consists of fields.
I assumed that you wanted to split one field into several others.