--export_fps values

It's posible to use a output fps value not included in the frc table?
I'm trying to use a 2 fps rate in my output file, but the filter always 
uses the input fps rate, i.e something like this (input at 12fps):

transcode -i input.avi -y xvid -J modfps --export_fps 2 -o output.avi

----
[filter_modfps.so] converting from 12.0000fps to 12.0000fps
----

but:

transcode -i input.avi -y xvid -J modfps --export_fps 5 -o output.avi

-----
[filter_modfps.so] converting from 12.0000fps to 5.0000fps
-----

Do I need to modify the libtc/ratiocodes.c to add my fps value?

Thanks.