Yes, look for a battery pack that supports pass-through charging. However, even if it does work, you might shorten the lifespan of the battery pack.

I had one of these http://www.amazon.com/External-UPGRA.../dp/B00ZDQFT6Q and I did something like what you mention. I used my dynamo to charge the battery while using the battery to keep my phone topped up. After doing that on a regular basis for a few months the battery pack stopped working. The manufacturer was nice enough to send me a new one, but I will minimize how much I use it for pass-through charging from now on.