I'm afraid I don't see the point in writing an application like that. If you want to install a deb file without using HAM then simply use a terminal with DPKG, it is a lot faster and probably less prone to problems than using FApMan. If you really want an application to do that then look into learning C++ (using the Nokia SDK) and research how to change MIME type registrations on the N900. You would also probably have to contact the people who wrote FApMan and get them to add a command line command for installing a deb package.