Small Business Server Backoffice in this context can be regarded as just the application that your NT server is servicing. For the purposes of the upgrade it can be ingnored.
You only need to provide the 2nd processor, then use the UPTOMP.EXE to update the Windows NT OS to a multiprocessor kernel and HAL. The util does not do it all so a few pointers to get you going...
MS KB PSS ID Numbers: Q142660, Q156358, Q168132, Q126608 & Q197896
Best of luck
-Hugh