Its the same as retrofitting ships. I remember in one of my games using starbases (2,5kt) with mining components. Using a retrofit series i was able to get the base full with mining components MUCH faster (i remember it took my 3 retrofits, though i packed the base full with plague bombs for maximum effience) than constructing them with a yard-ship could have resulted it. Its really awesome to mine a 200% mineral asteroid with 20 lvl 3 miners
Additionnally, the retrofitting doesnt stop the ressource gathering. Im sure of that as well.