Editing a cell....

Hi all,
I am working on an estimating program, and what I have is a drop down list of a particular product, when I select an item off that list it brings up the value in another cell, however if I don't have an item within my list I need to enter a custom item, naturally there shan't be any value associated with it as it is a custom item, thus I am trying to formulate the value cell so I can enter a custom value.... which is all great however if I want to reuse that value cell, now linked to the items which are in my drop down list it is no longer tied to a formula as I have over written the formula.
Is there anyway of automatically inserting data to a cell without entering a formula in it?
Or is there any other way to enter a custom value without having another column there.

Cheers..