Dear poison polisher,  I would be very careful about what you try to
mix with the shellac-you will almost certainly alter its setting/drying
characteristics.  Petrol- or oil-based material will not mix well,
if at all, so in order to work it will have to be spirit based, like
the shellac solution.

My advice (for what it's worth) is kill the bugs first with the _best_
poison available, then use the shellac as nature intended: unadulterated.
If you must try to kill two bugs with one stone (so to speak), test some
very thoroughly first to make sure it dries quickly enough, and is
capable of getting to the right surface characteristics needed for the
end result.

Paul Morris