Two options I can see, depending on your steel of course. If it is a tradional round profile metal steel then there appears to be sufficient room under the second row of slots (a darker layer) to carefully bore a hole to fit. If you don't have a drill press, then a chat to a mechanic or similar eqipped person may help, OR again assuming a metal steel embed a couple of rare earth magnets to one side to hold the steel. Most steels are either manufactured with or acquire a magnetic field, albeit a small one in most cases.
If you have a diamond or ceramic "steel" then sorry no ideas at the moment. Looks a good buy I got a slightly chipped ProCook block one from a cookshop at a vastly reduced price. I also have what was an absolute cheapy glass & metal one for my longer knives I got from TCM in the old days
