I’ve had a few people ask something similar to this, but I’m not sure how to do it just yet.
You could try a plugin like this – http://wordpress.org/extend/plugins/post-types-order/ which controls the post order, but it will most likely effect the post order of your blog as well……so that may or may not work for your situation.
I’ve probably mentioned this before, but I always recommend searching for an actual Portfolio plugin to handle things like this, as the portfolio template in the theme is really just a customized version of your blog. Adding that type of functionality would probably be too complex for the theme/template, which is why I recommend finding a plugin.
I would like to team up with some developers and create a plugin like this sometime in the future.