You'll be able to directly copy the folders from the PC formatted drive (NTFS) to a Mac formatted SSD - drag from the old drive to the new one - it'll have a green circle with a + to symbolize that it's a copy. In Lightroom you can either right click a folder and say 'Find Missing Folder' and point it at the new root folder of your photos, or you can say 'Find missing photos' and point it at one, then tell it to find neighboring photos.
What's the size of the external hard drives? Might be easier to start with a new external HD, copy everything to it, then wipe the now empty old one, and copy the second disk over.