Simple answer: "yes", absolutely.
Just don't use a cheap one... use a decent one from a decent brand. A cheap one can heat up, not provide stable power.
A 500mah will charge at the same speed as your laptop, while I believe that the Alien can charge up to 1Ah, so if your wall adapter is a 1Ah, it'll charge faster.
If you have a 2Ah (usually for tablets) adapter, it will still charge at 1Ah, it's the device that controls the incoming power.
Note that lithium batteries prefer slow charges (the harder/faster you want to charge such batteries, it will shorten it's life) as it keeps the batteries cooler.
Second note, as the mod is in series, one battery will eventually have a bit more power (as in series, the power follow through both batteries so one battery get more power/pushed through the other) so every once in a while, it's a good thing to flip them to keep them balanced.