I can't speak about Deff Dreads, as I'm a chaos player, but I'll give you a general idea (hope you find it helpful).
If you have a dreadnought, it has a base attacks profile. If you give it an additional close combat arm (to replace the ranged heavy weapon), it indeed does get an extra attack.
Yes, you do receive the extra attack for the charge, also.
Level me up! I, too, wish to storm the lands of mortals reaping what is truly mine...but for now, I am but a wee spot of energy...
"I embrace death without fear as I embrace life without regret."