I'm new to Raspberry Pi and bought a Pi 3 B model with a WD PiDrive.
I note that to install additional operating systems I need to use the NOOBS boot and overwrite existing data... Can this be done without overwriting data?
No, it cannot be done with NOOBS. NOOBS stands for New Out Of Box Software which says everything. Even worse NOOBS now has a special boot option that erases existing data when trying to install something new so don't try NOOBS if you want to keep your existing data.