Hi and welcome. I always use an external charger, and always recommend it. It can damage your batteries, or the mod, to use the built-in charging board, even if it's not absolutely guaranteed it will happen. A good charger isn't that expensive, really. You can get a Nitecore D2 from a US seller under 20 bucks, and an I2 even cheaper. I suggest it for several reasons, in addition to the fact some damage is possible: I think a stand alone charger does a better job of taking care of your batteries so they last longer. Using the USB plug on the mod for charging wears it out and it might not then be available for a software upgrade. It's easier and faster to just swap batteries in the mod. If you charge in the mod, some can't be used while charging, and even if it can you're tethered to the wall by the length of your charge cord. Using a stand alone, and at least two sets of batteries, will make sure you have something that can work at whatever point in the future one of your batteries goes south-- they all do, over time-- and you don't have to go without your mod until you can get new batteries.
Bottom line: I will not say "thou shalt not." I will say I don't recommend charging in the mod.