I guess you could. If this is a recurring event best thing to do would to set up a synced folder with your PC (dropsync or the likes). As soon as you annotate the PDF on your device it will update on your PC too. Now on the PC you can open the file and add PDF pages from another file to the end. Dropsync recognizes this as an altered file and will sync it to your device.

I've done similar things in the past (changing covers of PDF files) and it worked fine.

Good luck!

I have some PDF documents for a class that get emailed to me weekly and I was wondering if I can add them each week to the existing Boox-annotated PDF without screwing up the annotations (notes/drawings/whatever you call it)

Adding pages to a previously annotated PDF