Can I get a ghost pregnant?
Yes and no.
You can not impregnate a ghost while that ghost is tied but loosely to our world, but if she is brought partway back using the science opportunity, and made part of the active family, she can get pregnant and give birth as usual.
Follow-up:
Occult states in The Sims 3 are inherited, according to a fairly simple set of rules:
If both parents are normal sims, the offspring is a normal sim, irrespective of lineage in general.
If both parents are the same kind of occult, the offspring is the same type as their parents, and will inherit the occult details (such as wing shape, death type, etc.) from one parent or the other.
In all other cases, all occult-related inheritance comes from one parent or the other.
In all cases, only the parents are considered, and other genetics are blended as normal.
Unfortunately, ghosts don't work like other occults do; instead any child conceived by a ghost, or by a normal or occult sim in a ghostly state, will be a ghost, in addition to whatever occult states they might inherit.