I don't know the answer to your question.

there may be a way to go to the mysql tables, and export the text for all your posts into text files. You will need to know how to do mysql ...

On a related note: why are you leaving b2evo? or why do you want to export?

Have a look at Movable Type. What you want is a way to import B2evo. It has never been the other way around.

Good luck

I want to migrate to Wordpress
Can i downgrade B2evo?

You need to ask the place you're moving to how to migrate TO them. No one makes methods to migrate away from the app they're making. It's simply not logical to expect developers to work on something that a future non-user wants or needs.

Having said that, set your posts in feeds setting way high then grab your own syndication feed. That way you'll have a single file with tons of details about each post. MAYBE it'll help but I really have no idea.