I absolutely agree with RV.
If you intend to have what is typed into the combo-box displayed in the drop-down at a later stage, then you have to save it to its' own table. If you don't mind typing "Dr", "Sir" each and every time in the future, then ...
...you can set the "LimitToList" property to "No", which will allow you to enter your own values into the Combo-Box and this will be saved to the field "Control Source" for later use - queries / reports / etc.
I would still recommend RV's approach as it gives you better control over the data, with the overhead of one small table and an update form. I think it would be a small price to pay.