How can you combind plyometrics with weights?

Combining resistance training and plyometrics in the same session is called "complex training" and studies have revealed it to be superior than simply training one technique or another.

Complex training can be accomplished by intermixing plyometric exercises with resistance training OR by following a plyometric session with a resistance training session.

It is advised to lower the volume sets, and reps when performing complex training. You should feel fresh, and as if you are jumping at you very highest during a plyometric training session, and you do not want to be fatigued for the resistance portion of your training.

Complex training allows the central nervous system to maintain jumping efficiency, while increasing strength, and tendon elasticity, which results in a greater total gain in power.

While additional strength does aid in avoiding overuse injuries, especially to the knees, there is not a "requisite" amount of relative strength that must be obtained before undertaking a plyometric program. Essentially your sport involved plyometrics, and such a "requisite" would suggest that it is inappropriate to play your sport until strength levels have been reached.