I'll support what Bulgie said about being cautious. It is easy to mess up threads trying to add some more. Problems start with a die that looks okay but has a chip on it somewhere that can be disastrous on threads. Back when I started framebuilding/painting more than 40 years ago I mangled some threads trying to add new ones and that experience has made me very cautious ever since. Another problem can be that the chip from the steerer as the new tread is being cut, can get stuck in the die and when the tool is reversing to take it off, that chip can take chunks out of the thread.

What I do now (to prevent a repeat of the traumatic experience of my youth) is to fully back off the die every 1/3rd of a turn or so. I turn it far enough that it breaks off the new chip. I use plenty of cutting oil. I clean out the new chips in the die frequently with compressed air. Of course this blows out some of the oil too. I don't mind because I don't want to damage the treads that might involve a lot more work.

I've had a lot of steerer thread cutting dies in my career. English ones, Var, Park, Campy. They can get damaged pretty easily with aggressive handling. What happens is that a little corner on the die can break off making either cutting or backing out more troublesome.