We don't add subcateries for temporary uses, plus it wouldn't be very helpful anyway because categories on profiles are buried so far down at the end of the page that most members wouldn't ever notice it.
What I would do is to insert a note like the following at the start of the biography section:
{{Red|
'''This profile is currently being worked on by [[Gordon-4080|Gaile Connolly]]. Please do not make any changes at this time.'''
}}
Be sure to remove the note when you have completed your work!