Yes, you can definitely hire freelancers and assign other people to manage them.
One way to do this is to add othe Upwork users to a room within the Upwork messenger tool.
You may also use external tools such as Slack or Skype.
Furthermore, Upwork allows you to assign hiring and management privileges to other users. But you must do this within the Upwork framework. Every user. Including assigned managers, must use their own unique Upwork login.
If you are going to hand off management to someone else, I strongly advise a three-way conversation (or series of conversations, depending on complexity of the project) at the beginning, to be sure all three of you are on the same page re. scope of work, schedule, expectations about communication, etc. And you should be transparent with the FL from the beginning that they will be working directly not with you but with someone else.
Otherwise, there are lots of ways for something to go sideways.
For the record, MANY of the large projects I have worked on through Upwork involved one person hiring me, and another person serving as the project manager. So I saw one name as the "client", and occasionally saw that person in the Messenger tool. But USUALLY I spoke only with the project manager.
It’s also really common for me to work with teams through Skype, Zoom or Slack, but it’s really nice to know all of that before accepting the project who I will be working with/reporting to.