Application Support > Steam > Steam Apps> Football Manager 2020 > Data> Database > db > 2010 > edt> support_staff.edt
Open the support staff document with notepad or text edit or anything like that
It should come up with that below (v)
Copy one of them and change the name of the club, name and their job, and gender to what you want them to be and save it. Unfortunately it only works with new saves created after you do all that.
If you want to change photos of the staff I'd recommend https://www.mediafire.com/file/jmvm5hfzglsxy7o/Regen_UID_