It is possible to allow users to add values to lists as they are using the value list. With this option selected, users can enter a value into the field that is not part of the list and then add that new value to the list, so that the next time they use that list in that project, they'll see the newly added value in the list. While this can speed up data entry, it must be done only if you feel that the users will not make data entry mistakes. For example, if a user enters a value but misspells the entry, it will be added to the list in its misspelled form. If another user enters it in with the correct spelling, you will now have two entries, one of which is wrong. This can cause problems later in trying to sort or group by this field.
Allowing Users to Add Values to the List
Click Tools, Customize, Fields from the menu bar.
Select the field for which you want to define a list and click the Value List button.
Check the Allow Additional Items To Be Entered Into The Field option to allow users to enter values in the field that are not in the value list.
The Append New Entries To The Value List option tells Project to add these hand-entered values to the value list for future use.
The Prompt Before Adding New Entries option prompts the user to decide whether the hand-entered value will be added to the list.
Did You Know?
A value list is set to restrict the field by default so that it can only contain values that are in the value list.
Click Tools, Customize, Fields from the menu bar. 
Select the field for which you want to define a list and click the Value List button. 
Check the Allow Additional Items To Be Entered Into The Field option to allow users to enter values in the field that are not in the value list. 
The Append New Entries To The Value List option tells Project to add these hand-entered values to the value list for future use. 
The Prompt Before Adding New Entries option prompts the user to decide whether the hand-entered value will be added to the list. 
