Tomsol,
I’m the guy who wrote most parts of BlogML 2.1 and CS converter.
Dave Burke was kind enough to point me to here to help you. Currently there is no way to export WP content to BlogML except writing a custom theme which generates a BlogML file. Unfortunately this needs PHP background and none of PHP developers have done this.
I asked some of my PHP friends to write at least an exporter but they didn’t get back to me. We’ll be glad to get the help of PHP community on writing a converter for BlogML. Doing same conversion from MT is easy just by following the instructions for Blogger because they use same skinning method.
Although we (I and some other .NET developers) developed it and made converters for .NET blogging tools but BlogML is completely an open format (regardless of technology) and we say welcome to any guy who joins us to add something.
But in order to give you some thoughts I should say that you can create a custom WP theme which generates an XML file not a HTML file. What does this mean? Let me explain.
Instead of writing a <body /> tag for your theme you can put a <post /> tag for BlogML and so on. Same technique is done to export from Blogger to BlogML:
https://jeffbishop.net/archive/2006/05/21/690.aspx
But be sure we’ll try to find a solution for WP and hopefully WP community will help us to make a universal format to let bloggers choose their way. We’ve had many requests for this recently and I want to find a way.
Anyone who is interested to help us on BlogML to writing a converter for WP can contact me at: https://nayyeri.net/contact.aspx
Hope this helps ?? Feel free to contact me to get latest news.