Its Pretty Simple
You have to do nothing
when you bound your combobox from the database then for adding a new item into that combobox you must change the datasource.
Let's see an example to make it better.
Suppose cb1 is your combobox and the field name is District in Database.
when you bound it from databse it shows name of district, Now you want to add some run time values.
from Database you set the datasource like
cb1.DataSource=ds.Tables or cb1.DataSource=dt;
Go with the code as below:
DataTable Dt1=new DataTable();
again set like
This will add two new Districts named Delhi and Haryana in ComBoBox
This will solve your Problems
 Added code tags [/edit]