You can only program custom trunking tables via the included PC software.
1. Select the "Trunked Radio Systems" tab (if it's not displayed, enable it via the "Advanced" menu).
2. Select the trunked system to modify
3. Select the "Site Details" tab and click on the site to modify
4. Select the "Site Options" tab
5. Set "Trunking Tables Type" to "Custom"
6. Click the "Edit" button