Adding in percussion parts to a non percussion instrument

The most common way to notate this is to use x note heads. If you open the inspector (F8) the Head group will give you the option of changing note heads. You can select several notes at once and change all of them from the inspector. You can also do that from the Noteheads palette by double clicking the symbol. While the notes are selected uncheck Play in the inspector so you will not get viola music from this line on those notes.

For playback you will need to add another instrument. Use the i and make it whatever instrument you want except the drumset. You will need to go into the mixer (F10) and change the sound for that instrument to Hand Clap. Enter the notes for the hand clap on the staff this instrument. You may want to change the instrument name for future reference. To do this, right click the staff, click staff properties and change the instrument names as you like. You can then make the staff invisible through the Instrument Dialog (the i key).

It sounds complicated but it becomes easy if you do it a couple of times.