You have to treat those with a hyphen and those without a hyphen separately.
So:
Load the files into MP3tag.
Apply a filter to get those with a hyphen:
%title% HAS "- feat"
Select all files (Ctrl-A).
Create an action of the type "Guess values (import value)" for TITLE (see the FAQs /t/967/1 on how to create an action)
Enter as
Guessing pattern: %title% - feat %dummy%
Now filter again:
%title% HAS " feat"
Select all files (Ctrl-A)
Create another action of the type "Guess values" for TITLE
Guessing pattern: %title% feat %dummy%