Christopher,

The only thing I can think of would be to till and improve the soil
over the entire lawn, ignoring what is growing there. Get it ready for
turf, compacting it, etc. THEN allow whatever wants to grow to get a
good head start. At that point it would be ready for Roundup. After
the Roundup takes effect, *without disturbing the soil* put the sod
down and water as you normally would.

This sounds like a lot of trouble and it sure would be. The only other
option would be to have someone entirely remove the top few inches of
lawn and soil and start over, but you never know what you get with
purchased topsoil.

Or you could do what I do, which is to keep turning parts of the lawn
into garden, gradually, and learn to ignore crabgrass. My philosophy
is "If it's green, it's a lawn."