The W-87 Peacekeeper warhead (to be redeployed on the Minuteman-III) has a current yield of 300 kt, that can be increased to 475 kt by adding a HEU sleeve or rings to the secondary. Whether this represents an actual addition to the existing secondary, or whether it replaces an existing unenriched sleeve is not known. The W-88 Trident warhead is a closely related design, and has a current yield of 475 kt indicating that it is already equipped with this addition. The 175 kt yield difference amounts to the complete fission of 10 kg of U-235.
Now, once one considers using substantial amounts of HEU in the secondary, the question of why the fusion fuel is needed at all arises. The answer: it probably is not essential. The idea of imploding fissile material is what set Stanislaw Ulam on the path to that led eventually to thermonuclear weapons. But with the availability of large amounts of HEU, and the trend toward smaller weapon yields (compared to the multimegaton behemoths of the fifties), the Ulam's idea of using radiation implosion to create a light weight high-efficiency pure fission weapon returns as a viable possibility. It is an interesting question whether all modern strategic nuclear weapons *are* in fact thermonuclear devices!
