This assumes you already have a featured artist as part of the <Title> tag and that it appears at the end of the tag. Such as
This also assumes that you have installed boroda's Additional Tagging & Reporting Tools plugin (https://getmusicbee.com/forum/index.php?topic=3833.0
1- Start with a couple of test tracks. DO NOT do this on your entire library until you are certain that the results meet your expectations.
2- Create a backup, or a copy, of those test tracks.
3- Test to make sure the backup or copy can be restored.
4- Test it again.
5- Change your main panel view to tracks view.
6- In the search bar, type "(feat" without the quotes.
7- All the track titles listed should be in the format as shown in the screenshot above.
8- Select the tracks you want to work with and right-click. Select Additional Tagging Tools > Advanced Search and Replace
8z- You can also reach Additional Tagging Tools from the Tools menu.
9- In the AS&R search bar type "feat" without the quotes. The results will give you four presets.
10- Select the last one. It should say "Extract featuring artists from track title and add them to the (Guest) 'Artist' tag..." DO NOT CHECK THE BOX TO THE LEFT OF THE PRESET.
11- In the box next to <Custom text 1> enter "feat." without the quotes.
12- In the box next to <Custom text 2> enter ";" without the quotes.
13- Click the Preview button and your tracks will appear below with the original and new tags.
14- If you're happy with what you see, click the Apply button.
15- Close AS&R and open the Tag Editor. Click on the dots to the right of the artist field.