For example, for managing a disk partition for another system where the user exists. I know I can simply create a user temporarily but I find this question interesting.
Yes, you can
chown to a numerical UID that does not have a corresponding user.
chown UID:GID fileName can be done either with numbers or username or groupname
chown 1000:1000 dirname is valid
you may have to reset the directory permission with
chmod 755 for example after doing it to get access on it