Adding a (series) switch to a timer
A three-way switch is not necessary. You can connect a single-pole single-throw in parallel with the spring timer.



EDIT: Future readers should note that this is electrician-safe only because both switches are in the same gang box. With the switches in separate boxes, someone working on this circuit in future might be surprised to find voltage on what he thought was a load wire that he had just switched off.

Also note that this is equipment-safe only because a simple mechanical spring timer can tolerate hot voltage on its load terminal. (In fact the spring timer probably does not have "LINE" and "LOAD" labels.)

If anyone ever wants to cobble together a similar setup with a timer that (1) doesn't have an "ON" setting, and (2) may not tolerate hot voltage on its LOAD terminal, then he will need a three-way switch. But it can't be wired according to your proposed circuit. Instead, the timer must be wired between the mains power and the three-way.



Here the three-way selects between the always-on hot or the timer switched hot. It never feeds power back upstream.