You would have to assign primary ownership to extension 10 for all the lines (#208). Then program remote call forward for x10. Then you program speed-dail 80 for x10 for the number you want to forward it to. Put a forward button on x10 and then at night they would press it followed by 10 80 (forward 10 to speed-dial 80). In the morning they would press the forward button again and press 10 10 (to forward calls back to 10.
This will only work if it is ok to have ownership of the lines assigned to x10. If they need an attendant (vs. x10's mailbox) to answer during the day then this won't work.
You could leave off changing line ownership if they need an attendant and then have callers press 0 to route to x10 which would then forward off. It's definitley not the cleanest thing in the world, but it would work.
Also to remember is that a forwarded call occupies the line it came in on as well as the line it dials out on for the duration of the call. If they have POTS lines, it might be better to have them forward line 1 through their provider at night versus trying to do it through the system.