You either have to manually drag the notes on that track longer, or you could mute that track and export the rest of the song. Then export "that" track and none of the others, and then use e.g. Audacity to make "that" track longer and stitch it to the rest.
Very interested!!!
Also, when they fix the register thing. Maybe you can upload the theme on the homepage?
http://lmms.sourceforge.net/lsp/index.p ... I%20themes