Username or EmailPassword
I'm not exactly sure what "Stow" is, but if it's another package manager, you may very well be able to replace it over pacman. I don't imagine it being too difficult. If it's a repo to download packages from then yes you can add it. You just need to make a line in /etc/pacman.conf for it. It would look something like this:
[shadowhand] #This being the name of the repo
Server = http://arch.os-zen.net/pkg/shadowhand #This being the server address
Then go ahead and run pacman -Sy and then it should update that repo for you.