Would it be possible to take the signal from a passive parabolic/helical WiFi antenna (2.4g) on the hill between to the two buildings, amplify it if needed, and send it with a directional antenna to the other building, which would also have an outdoor directional antenna?
To clarify, parabolic antennas and many helical antennas are directional in nature.
Is it possible? Certainly, many things are possible. Odds are that Wi-Fi will not have the power to provide good service within legal limits and you should look at dedicated point-to-point wireless solutions.
Additionally, if this is possible, could it be done with a passive configuration, or would a repeater and possibly some type of filter be needed at the mid-point?
Again this is possible, at least from what I know of RF theory. You could connect two antennas directly together, and this could work in both directions. However, the realities of the situation are that it is likely not to work as well as you might hope.
In the first place, you would need a signal strength that was powerful enough to travel the distance (>1km if your two buildings are 1km apart). This is to account the distance added by the elevation changes as well as the signal loss you will experience without amplification.
Second, you would have to properly align (and keep in alignment) all four antennas used for this purpose. Having any of them out of alignment will further reduce the signal between the two end points. Getting highly directional passive antennas into proper alignment will be a challenge.
Third, any such solution will also pass noise/interference between the endpoints that would otherwise be blocked by the hill. This point would be even more applicable using any sort of amplification without correct/adequate filters.
If you do amplify the signal, this would have to be bi-directional amplification and the amplification would have to be filtered to not amplify noise/interference as well. This also means that you need power at the midpoint. If you are already putting power at the midpoint, then you are probably far better setting up two separate point-to-point wireless links and bridging them together.
Alternatively and possibly best (depending on the size of the hill) would be setting up antenna masts on each end and elevating the antennas to a point where the two end points can directly connect to each other. Antenna masts for point-to-point directional antennas is preferable in any case as it helps to avoid obstructions and RF noise.