Using an old router as a repeater is possible. The issue and legality of your request is what comes into play here. It's someone else's internet, a business connection at that and they've given you special permission to access their connection using wifi ONLY. Circumventing their permission to add/extend this connection can only end badly.
That being said, why not just get your own connection from same provider that the business is using?
That being said, why not just get your own connection from same provider that the business is using?