The other mercury bullet thread...
(Tha's a link, son -- don't know why my browser isn't highlighting it this morning, but it does change color if I mouse over it)
The usual way of alloying metals with wildly different melting points is to melt the lower melting metal first, then dissolve the higher melting into the pot. With mercury already molten at room temperature, you'd most likely add the lead to the mercury in a cold pot, then heat just until the alloy is melted before casting your boolits. The alloy almost certainly melts at a lower temperature than lead (which, as noted above, melts at or just below the boiling point of mercury), and starting with amalgam would let you get the melt going and make up your alloy without boiling off the mercury.
That said, I have to agree with opinions above, that making boolits with mercury is generally a bad idea, when a slightly larger amount of (much cheaper and vastly less hazardous) tin, or even smaller amounts of tin and antimony together, will do just as good a job of hardening boolits. Personally, I'll stick to wheel weights...