I don't have an exact value for you; Halliburton would be the best place to go, but I seriously doubt you can do any damage by "over-pressuring" the pump.
If the pressure going into the suction manifold gets above a certain level, it will overcome the force of the spring holding the suction valve closed, lift it off of the seat and pressure up the pump chamber. It will then try to lift up the discharge valve against whatever pressure there is on the other side. If it does lift discharge valve, then the charge pump will just flow through the HT-400 without causing any damage.
If the discharge line is closed or has a larger pressure on it than the charge pump can overcome, then your pump chamber will see whatever the deadhead pressure your charge pump puts out. That is not going to be a problem since the pump chamber is designed to see the full discharge pressure that the HT400 can put out (surely much greater than whatever your charge pump can put out).
Or is it? Certainly you are using a centrifugal to charge the HT-400 and not another Positive Displacement pump?
Can you share any detail on your charge pump setup? Type pump, rated flow & pressure? Is it setup to turn on and off with the HT400?
I can't imagine a few hundred measly PSI doing anything to that massive chunk of iron which is the HT400.