Hi John,
only the Expansion Memory (where the Wave data of the installed Expansion Voices is stored) will be formatted when you install a Pack Install file (.cpi or .ppi). So your User data stored in the User drive or on USB stick (e.g. Registrations, Styles, MIDI Files, Multi Pads, User Voices, User defined System settings) will NOT be affected when installing Expansion data.
In short: You will NOT loose any data in this case (except the previous installed Expansion data).
Best regards,
Chris