No, not directly - the USB port is not user accessible. The best you can do is to get NAS to serve the files from (I know that Synology NASes have a Plex server) and place it in the same room as the TV so you can run an Ethernet cable directly between them. Whether this is feasible depends a bit on your setup - if you get your Internet access over cable and have the cable modem at the TV anyway, you might place the router there and connect both the Apple TV and the Synology to it by cable.
Otherwise, I can only suggest that you try to improve your Wifi coverage to make the streaming work