Enrico,
I don't know if there is a script for it, but, basically,
it would require 1) msgcat, from gettext, to concatenate all
desired PO files together in one big PO file; and 2) po2tmx,
from translate-toolkit, to convert this big PO in a big TMX
file.
Allied with gmirr.sh or mt.sh (both provided in this
thread), I believe a simple script preforming the above task
might do this job easily (added to my TODO list)
Cheers,
Rafael Ferreria