What if I want to combine Ctrl+Shift+1+0 to obtain "≠", for example? I'm saying this combination, like I could say any other (like a+b), it's just one example.
I don't have the information regarding xmodmap, but here are two other options which I am using if you are interested.
With the compose key set, you can hit / = to get ≠. Or you can set custom sequences in .XCompose, but I don't believe you can get the exact combination you suggested.