I guess you’ve figured out by now… but in case you didn’t…
First, you have to open the folder where all your animations are (I recommend you have a separate folder for your animations and your skeletal meshes and skeletons, or it will detect the skeletal mesh and ask you to replace it without showing you the import window).
Then get .fbx file with the new animations and make sure it is named exactly like the file you imported the first time, (also make sure your skeleton hierarchy and number of bones are the same, or the animations won’t import correctly)
then click import and select your file, one the import window, uncheck import mesh and below you’ll see a blue square, there you have to select the skeleton you already have in your file (the first one you imported), and hit import all, and all your new animations will be in the folder, though you may have to rename them.
I hope it helps.