If its an ISP supplied router, especially a modem router or they have some additional control over it - it is possible. If its your own router, practically short of having someone wander around checking, its unlikely.
However, some ISPs might suspend service due to "suspicious" activities or send you nastygrams.
On your own router or routers - it should be impossible for the ISP to see your wifi password (it would be a nasty security hole), nor should there be any reason for them to need your wifi password (so they shouldn't need to know it, or ask for it).
Even with many ISP supplied routers, the password is randomised on a router in a sealed box so it is unlikely the ISP will know what it is.
Any service that sends your password to a third party is a terrible idea. People sometimes don't realise things are terrible ideas. If you suspect your ISP is doing this, you may want to check, both online and with the ISP what their services are.