pm(1) only takes local files as arguments, it is not meant to deal with external repos of some kind.
I have written two small shell script, namely repo(1) and repogen(1) to deal with remote tarball fetching or whatever. I've listed some examples in pm's README, but for all the lazy asses in there (I know there's a lot of you!), here is an idea of how it works.

Let's say you have two machines, "server" and "client". On the server, you'll have the build system of your choice, having all tarballs created under /var/www/dl.z3bra.org/releases. You'd use repogen(1) to turn this directory into a repository (it simply generate a .list file, with the name, version and checksum of all tarballs):


Now that you have your remote repo setup, you can sync it locally using the repo(1) utility:


Syncing the repository will NOT download all the tarballs. That would be a huge waste of time. You can list the content of your local repo with "repo -l". This will give the same output as when you sync the repo, minus the checksums.
To actually download a tarball, simply run repo(1) with the name of the pack you want. Once done, the full path to the tarball will be printed to stdout:


If you call the command again, repo will check the tarball's checksum against its local .list file, and if they match, will output the full path again to stdout, without redownloading the file (this makes sense right?)
You can then use repo's output to feed pm(1) without having to know where the tarballs are located:


is equivalent to:


At some point, I'll try to make pm read path from stdin, leading to a simpler way to install tarballs ;)
Repo can already read pack names from stdin, leading to some neat one liners! For example, to download all packs except the libraries, you can run the following:


Note that this is still a work in progress, which is why I didn't commit neither repo(1) nor repogen(1). I'm still trying to figure out how they should work, and what the workflow should be.
If you're interrested in giving it a shot, and reporting some points that might be awkward or incoherent, just ask and I'll paste the scripts ;)