Cranberry Stout

I want to brew an all-grain chocolate-cranberry stout as a Christmas beer. I have brewed with fruit once before (raspberry wheat ale) by adding fresh frozen into the secondary with great results.
This will be my first porter. Can I add fruit in the Mash, or introduce juice into the boil, or just throw cranberries into the secondary as i did with my rasberry wheat?

I would love to hear your suggestions

I would just throw it into secondary. That way you will get the most flavor as it would not have boiled off in the boil, or been fermented out by the yeast in primary.

definitely secondary, although one other option could be bottling with the juice.