Hi Franco,
Composer currently only supports tuplets of 3 and 5. However, since the 6 tuplet is equivalent to two 3 tuplets, perhaps it would suffice to write it as such:
Sextuplet_asDoubleTriplets.png
When played it will sound the same, and any "live" musicians should be able to easily read the two triplets in line.
ttfn,
Sherry