Hello everyone, I have this lovely old Dura-Ace wheel, but it has this annoying very obsolete 6 speed freehub with the top cog that threads externally. Can I upgrade it to 7 speed freehub? You cannot fit a 10mm allen key inside, this is a weird different spline interface, anyone know which tool fit?

Yes, but you need to track down the long out of production TL-FH10, then convert it to the 8-speed hyperglide body from the FH-7403 hub and longer axle and NDS cone from that hub as well. Then you would need a spacer for the 7-speed cassette on the 8-speed body, and likely will have to space out your frame as well.