Does the connection have to resist any lateral load?
Hilti fasteners - X-DNI pins
These have some capacity and it depends on the thickness of the tube walls.
Or threaded studs can be welded to the HSS and the nailer can be attached with nuts. The nuts can be countersunk into the wood nailer if a flat surface is required.
If powder actuated fasteners don't have enough capacity for what you are doing, Lindapter has fasteners called hollo-bolts and lindibolt 2. These are similar to sleeve anchors, and are designed for hollow steel sections. I don't know what your loads are or what your application is, so I have no idea if these Lindapter bolts are appropriate for your condition. Powder actuated fasteners would probably be much cheaper and would be the first choice.
I've also just used good old thru bolts. ITW Buildex also has wood to steel tek screws. I don't know how much capacity you need out of your connectors.
