Add permission to Mailbox (Calendar) without overwriting existing permissions

I’m trying to write a powershell script to add Reviewer permission so every user can view any other user’s Calendar. Some users allready have read and write permissions on certain calendars. Am I right in assuming that the ‘Add-MailboxFolderPermission’ cmdlet only adds Reviewer permissions to those who have none, but leaves the ones with read/write permissions alone? This is for Exchange 2010 btw.

(Excuses in advance for my limited knowledge of powershell, hope I didn’t make an ass of myself asking this question)