Yes to both... sort of...
You could use Remote Programming feature of the system to turn it on
(Assuming default FACs)
On your phone dial:
359 (Remote Programming feature code)
<ext #> (The extension you wish to "program")
<Ext password> (This is set in system programming via Session Manager, often the same as ext #)
372 (DND Toggle)
Press Speaker button
<System will show current state of DND>
From Phone Manager you would log in with that user's credentials and turn on DND.
From the User Web interface to the 5000 you can turn on DND by logging in with that user's credentials.
If you have the PC Attendant Console, the attendant can set/clear DND for users.
From Session Manager it can be done as well, but it involves using the Online Monitor, and if you are not familiar with that I suggest NOT attempting it because a mis-step or accidental change in the Online Monitor can have serious implications.