[AccessD] Add new data to existing table

I want to apologize ahead of time for my ignorance of Access but I am new to 
this and would appreciate any help that would come my way. 

I have a Customer entry form whereby I am entering customer first name, 
customer last name, address, zip code, City, State and remarks.  I also have a 
ZipCode table where I have  Zip Code, City, State fields. When entering data into 
the Customer Entry Form the Zip Code field goes out and looks up the City and 
State and returns the value from the table on exit from the Zip Code field.  
My question is this:  when a value is not found in the Zip code table that 
matches the entered zip code, how do I from the Customer entry form add the new 
Zip code, City and State to the Zip Code table?

Again, I appreciate any help you can extend me.

Doris