How to import (many) FIT files into Basecamp

Hi, I have downloaded from my Garmin Connect account 100s of FIT files that I accumulated and stored on my PC over the years. I would like to show them all on one (world-) map (generate some type of heatmap). I think that can be done in Basecamp on my PC, but how can I efficiently (as opposed to one by one) important 100s of FIT files?