This needs some requirements. First of all you have to have enough experiences with software development and the technology that is used for your source and destination blogging tools.
If you have this requirement, then you can check out current BlogML tools and read our specification and documentation (coming soon) to get started. The rest is your skills to be able to develop such a tool but don't hesitate to contact us and ask your questions. Also let us know if you create a BlogML tool and we will support and promote it.