If I machine a cone-like piece of ferrous material, would the narrow end have more magnetism than the other end where the magnet is placed?
Any ways to “concentrate” magnetism?
ISTM your truncated cone is a good way to increase magnetic flux density. The same flux through a smaller area gives a stronger magnetic field. There are other interesting ways to “concentrate” magnetism such as your basic explosively pumped flux compression generator 
The flux capacitor works better. Only it concentrates the flux in a temporal direction. 
It might work better to use a good conductor, or even a superconductor, rather than a ferromagnetic material.
And I know for a fact that there are devices for creating a super-high magnetic field that work by explosively compressing an already-strong field. And yes, I do mean “explosively” in the literal sense of the word, as in they use TNT or another similar material.
I think this will only work up to the saturation level of the metal you are using to make the cone, which would be pure iron (or the lowest carbon steel you can find), and this is no higher than neodymium iron boron magnets can already have at their faces. They can have about 1 to 1.3 teslas of magnetic field intensity I think. Now, if you make your pole pieces of holmium, and you keep it cryogenic, you can get higher. But not at temperatures practical in a small shop, I think.
You could probably get higher using a copper solenoid, but maybe not continuously. If you run really high current through a coil you can get there, but you will overheat the coil soon. So you have to switch the power on briefly, and then give the coil time enough to cool down before you try again.