As the old saying goes, "It's never that easy." A 6mm-0.75mm is almost the same thread pitch as the 1/4-28 UNC. The 0.75 thread pitch is 29 threads per inch. It will start but not seat and will be loose in the threads. The 1.00mm thread pitch is 25 threads per inch. If one does seat a 6-0.75 in a 1/4-28, it will eventually get blown out. It's far better to verify than rely on my posting on the Forum.
The Allen/Uberti Hawken was built to US specs originally and likely had US standard threads. Pedersoli continued with the 1/4-28 nipples after it acquired the Uberti Hawken.
However, it's best to check. If your local hardware store is leery of someone bringing in a gun barrel, you can buy single bolts of the three standard nipple threads. Get a 6-0.75, 6-1.00 and 1/4-28 machine screw and see which one is the correct fit.