Should, as in "they should be able to pick up the spell on the way even if they don't already know it".No, they shouldn't. They might be able to. Mathilde started Journeying with two Relatively Simple spells, and didn't fill out that section of her spell list until after becoming a Magister.
I feel this should be some kind of general notice to the thread: stop hoping/expecting/planning around All The Shinies, like figuring Journeymen "should" know lots of stuff or speculating about how the Journeymen might deal with the Skaven by picking up Battle Magic.
At least, based on our own experience with Relatively Simple spells. Furthermore, they know they are going to a warzone, and would have studied appropriately.
I sure hope the Jade Journeyman didn't join a war while packing only the beer spell for instance