I already got B-Kyurem and W-Kyurem.But can they learn the power up of Fusion Bolt/Fusion Flare just like Zekrom/Reshiram?
Yes because I've looked at both the moves and it doesn't say that they are Zekrom and Reshiram exclusive.
If by power-up you mean Bolt Strike and Blue Flare, then the answer is no. But they can learn Fusion Flare and Fusion Bolt.
Hope I helped! :)