Hello,
the sum input options apparently has been "lost" with lpm_mult Megawizard V7.0 or V7.1, it was still functional with V6.0. But the library lpm_mult (.v, .vhd,.tdf, whatever you prefer) still has it. Thus you can use it by manually instantiating the megafunction. You should consider, that this implies a logic element multiplier, hardware multipliers don't have the partial sum feature.
Personally I prefer infering multipliers from HDL, which saves thinking about optimal implementation for a particular device. So I was never aware of the change to Megawizard.
Regards,
Frank