Once you have OLCIed, you are not able to pay for extra baggage online.
Maybe they would be able to "uncheck" you to enable you to do this, but it is not normally possible. I certainly have tried and failed when I was accidentally checked in (it was an odd situation where all I did was to change the seat and it was treated as my having OLCIed - perhaps I pressed something I should not have).

This is the case with a few other airlines that I know of.