Try an action of the type "Replace" for whatever field that applies (looks a lot like the filename to me)
Search string: (ft
Replace string: (ft_
where you use a instead of the _
Perhaps you even extend the replace string by "ft. " as the "ft" is an abbreviation.
This replaces all "ft" preceeded by a bracket. If you have more of the ft-cases then either leave out the preceeding character or use a different one.
I am sure there are numerous occasions where the ft is part of a normal word. Like "Taylor Swift" or "gifted" and so on. So: caution.