From the scheduling workflow click the "Change" button next to the piano you want to add the service too. In the search box for services type the name of the service you want. If whatever you type does not match a service item located in your Master Service List, then Gazelle will give you the option to "Add Misc Services".
To add the item give it a description, duration, and price. Hit "OK" to add the misc service item to the piano and then "Ok" again to add it to the booking workflow.