To try and understand your Question, I have reviewed the Creating a Python Add-In Combo Box help page to remind me that ...
A combo box contains an editable field and a drop-down list. The user
can select a value from the drop-down list, which appears at the
user's request. If you make the combo box editable, the combo box will
include an editable field into which the user can type a value. The
scale combo box in ArcMap is a good example of a combo box.
If you are coming from a VB.NET background, then you may be thinking that a Python Add-In Combo Box can bind to an MS Access database as easily as a solution like these from another forum.
I have not had a requirement to try and work out the steps to do this using a Python Add-In Combo Box and ArcPy but am confident that Accessing data using cursors from the Data Access module of ArcPy (arcpy.da) will be required.
The example on the Creating a Python Add-In Combo Box help page does not include an editable field but that code could be:
- Run through to learn how to update the combo box with the list of layer names
- Adapted to learn how to update the combo box with a list of field values instead by using arcpy.da.SearchCursor
- Adapted more to include an editable field and work with arcpy.da.UpdateCursor and arcpy.da.InsertCursor
If there is existing sample code available for including an editable field in a Python Add-In Combo Box and using it to update a Personal Geodatabase feature class then I am not aware of it.