The best way to handle BTrieve, if you don't have the application itself, is to purchase an ODBC driver for it and use that to either import or attach the data.
The problem is: BTrieve is a very primitive system by today's standards (albeit very fast), and controlled by a company that knows exactly what its property is worth. So you probably will have to pay $'s. But this approach does work.
If you do happen to have a BTrieve native tool of some kind that can export the data into a common format, such as XBase, then that approach works too.