I think its a case of card or usb won't make a difference. However you'll need to disable LAN for WLAN to work and vice versa. IE, disconnect the Ethernet connection for wireless or disable the wireless for ethernet connection.
My son has his email as secondary through my BT Account. He has a let through a company that will not allow internet access through any other provider other than themselves at extortionate rates. As he is on a BT fon hotspot I wondered if he could get round this by using my email and sign in and then signing out and back in as his own email. He has no internet access at the moment as it would cost him a fortune.
If he is using a Laptop or PC that belongs to the firm he works for you should be able to understand why they do not allow just any connection. So it would be wrong for us to help with suggestions to work round the Admin settings
Report this "company" as soon as you can to BT. It is a hijacking malware. As he uses the internet under your account be aware that you might also be under threat. Ask him to go to control panel, internet options, connections tab, Lan settings and untick the box for 'Use a proxy server for your Lan....).
Use the BT's security program to run scans on his Laptop but report the issue as soon as you can as he is using your hotspots connections.
He is not working for the company that owns his flat. They will not allow BT or any other provider to give internet access. He has no choice but to use them at an extortionate cost or have no internet provider
If I answer the basic question.... on all the computers I have used, wifi and ethernet (wired) co-exist quite happily. The wired one, if connected, will generally take priority. Therefore suggest you install usb dongle, disconnect ethernet cable and try and connect to BTFON.