In the software it will export the template buttons,
but you can get round this by setting the button sizes to match those you've created.

Export as usual and then replace the exported images with your own buttons.
Provided they're the same size you'll only have to worry about getting the filenames correct.

If you don't want mouseover states then just save the one button as both filenames,
eg. yournav_b1.gif and yournav_b1_over.gif