You'd probably need to extract the mod(s) in question, using the extractor which can be found under #Modding-support on the Discord channel that Raze linked.
Then you'd want to look into \Public\ModYouWantToModify_UUID\Stats\Generated\Data and find the appropriate entry for what ever skill you want to modify the icon of, and duplicate those entries with the icon entry being replaced with one of your own from your new mod and your atlas, and make sure that mod loads after the overridden mod so that your icon is the one who writes that entry last.
I'm not exactly a stats guy so I can't say for certain, but I believe this would be the way to do it. Or one way at least.
Joining the Discord is definitely a good bet though, a lot of people there with way more insights in that aspect of modding than I have