This is easy with GPSmapedit too. You can assign Postal Adress Information for searching the intersection of POI in GPSr

First you have to enter information about country, Province and City in gpsmapedit using toolbar Tools->Edit Postal Address Items.. When popup window opens , enter the country,province and city information. Then go to toolbar Edit->Select->By type..

Popup window opens and Select poly-line category which you want assign City Index. Click OK and all poly-line of selected category are selected in main Window. Now right click on selected poly line and in pop up menu click on Modify ->Postal Address-> City.
You are done. All selected objects either poly-line are POI have city index for searching in Garmin units.