EXTRA BAGGAGE

My flight ticket from Beijing (PEK) to Stockholm (ARN) allow for 1 piece of baggage. I'm in Economy class.

1)Can I still bring an extra bag (2 bags in total) for check-in without pre-booking that?
2)Also, I tried to find what the price per kilo would be for overweight on one of the bags, but couldn't find it.
3)What does it cost to check in an extra bag?

Thank you for the help!