From my FCOM...
The flight crew can arm the CLB mode during the takeoff, go-around, climb, and cruise phases and engage it during the climb and cruise phases.
The CLB mode is armed:
In flight, when the climb or go-around phase is active, and the following conditions are met:
• The lateral NAV mode is engaged
• The FCU selected altitude is above the aircraft's present altitude and the aircraft captures or flies an altitude constraint.
Which means.... If you select NAV before GA ACC ALT, then CLB will be armed, and will engage at ACC ALT.
But.. you cannot engage CLB manually in GA flight phase.