Is it possible some comets that enter our solar system came from another planetary system?

Very unlikely. The distance is usually too great for that to happen, and comets tend to form along an orbital pathway around a specific star. This implies that all of the material that formed the comet has come from the same system. Since star systems are so far apart, it is unlikely that the comet came from that system. Also, it is unlikely that the comet will escape its system and make it here intact.

Unlikely. There is very little chance that a comet from outside the Solar System would have a velocity comparable to a body formed within the system. We'd expect an hyperbolic orbit for any extra solar body.

.

Maybe. All the comets that we know of comes from our solar system. But there might be comets that came from another planetary system, but it's very unlikely. But so far, we don't know yet.

Most, if not all originate from within the solar system, it is highly unlikely that any comets come from outside the solar system since the distances between stars are vast. It would be like randomly shooting few bee-bees from Sacramento, California and hoping that they land in London.

All comets originated from outside our solar system because it didn't exist when a lot of those comets were still zooming around. Some do come from the outside, but it is very rare nowadays.

I don't think so. Because they have work against the gravitational force to escape from it. If they are inside the infinity point of a gravitational field they have to escape by certain energy which they don't have. In rare case it may happen.