Get-CalendarProcessing - BookInPolicy

Sure it's easy but has defeated me so far!

If I set the BookInPolicy on a Room Mailbox for a couple of email addresses -

Set-CalendarProcessing -Identity "Room9" -BookInPolicy "tom.clark@test.com", "paul.stones@test.com"

What's the most elegant method (using Powershell) to add "pete.smith@test.com" to the two existing addresses?

Set-CalendarProcessing -Identity "Room9" -BookInPolicy "pete.smith@test.com" replaces "tom.clark@test.com", "paul.stones@test.com" with "pete.smith@test.com" rather than appends it to the list (which is what I want)

Thanks in advance