Your question is not clear: do you want to connect a NAS (Synology for example) to your bbox ? In that case, it’s via ethernet that you need to connect it to the bbox,  not USB. Or do you want to turn your bbox into a NAS by connecting an external hard disk via USB to the bbox ?

id like to turn my bbox into a NAS with an external drive

Although I did not test it, it seems possible: https://www.test-achats.be/hightech/telecom/news/tirer-meilleur-parti-votre-bbox

But I am quite sure that the Bbox does not provide all the features of a NAS. It should be limited to file sharing. Probably only with the smb protocol.