I think you meant open center system, not closed center system. You CAN tee the pressure line of a closed center system, to add a mid valve for a loader. That's how JD recommends, and my closed center system, JD 2030 mid loader valve is plumbedMM,
In general, you cannot Tee any limes except low pressure return to tank lines in a closed center system. If you have two inlets/outlets, or tee the PB lines, fluid will "take the path of least resistance" and that would be the path that you are not commanding any work from.
Having said that, I'm not sure I'm understanding what you are thinking of doing?
(also, moving thread to hydraulics forum)