Copy Other AE arcs?

Only if it is someone you know, they save the arc locally, then send the arc's files from their PC to yours (by email or file sharing, etc.) - then you could add the files to your CoH folder and work on the arc as if it was one of your own saved arcs. I believe this would work, though I haven't tried it personally.

Otherwise, no - it would just be too easy for people to plagiarise other player's hard work, make a few cosmetic changes, then claim all of it as their own work.