You should be able to modify an existing aircraft by adding 'floats'. This can done by adding the needed lines to the [contact_points] section of the aircraft.cfg file.

You can download the FS2004 Aircraft Container SDK here-http://scenery.org/tutorials_fs2k4_SDK.htm

It outlines the aircraft.cfg and will explain the elements of the [contact_points] section.

The info is valid for FSX also, if that's what you're using.

Dave