Best way to acheive this is to base your combobox on a table and add the value to the table. If you search the forum I am sure you can find a sample....
You would be using the "Not in list" event, trapping the new value. Then using a recordset to add the new value to the table.
Most of these comboboxes have a very limitted number of values (up to 10 or maybe 20?) I have used 1 table in the past for all my comboboxes using an extra field to filter out the values I need for that 1 combobox. This instead of having a seperate table for every combobox, which i have seen people do!
Regards