No, you can not tee into an open center circuit where the fluid is routed through all the valves.. If you tee in, the original path is still open to tank. However, if you place a shut off valve in a certain part of a circuit, then the fluid is routed through your grapple valve, and it will work.
Why not use a diverter valve, solenoid control, a switch on the handle switches between the curl and the grapple.
Why not disconnect the BH, and plug your grapple valve into the QD's where the BH plugs in, and everything will be correct.