I thought exporting foliage as glTF would be a feature in the 5.4, but it seems it is not. I found a workaround, even if I am not sure about the quality you’ll get. Need to try.
Anyway, select your actor foliage once you set up some foliage on your map, then go to the tab “Actor” in the upper pannel on UE, then “Merge Actor”. That should create a static mesh from your foliage. Place your new SM_Foliage on the map, try to make it fit the position of your actor foliage. Then delete your actorFoliage. And then you should be able to export all your scene and get it in Blender.