You can't.
However, you can send incoming calls directly to Mailbox 10 (or 11, or whatever mailbox you choose), bypassing the auto attendant all together. Make the greeting in Mailbox 10 say "Thanks for calling ABC Company. We are open from 8am to 5pm, Monday through Friday. If you've reached this recording during those hours, we are assisting other clients. Please leave a message, yadda, yadda, yadda." Put a D.N.D. button on Ext 10 and treat it as if it were a Night Service button, for Immediate answering. One mailbox to cover both Day and Night situations.
To do this, #206-7-(Line Number)-2, #208-(Line Number)-(Covering Mailbox Number), be sure that the mailbox has Auto VMS Coverage #310-(Extension Number)-1 and has Immediate or Delayed Ring for the lines it will be covering. Use VMS Coverage Rings (#321) for the number of rings before answering in the Day mode.