There doesn't seem to be a way to store constants that have already been defined. This function, apc_define_constants(), does both things: (1) defines a constant, and (2) stores the constant in the cache. This is unfortunate, since it introduces the requirement to handle constant definitions differently when APC is not available.
(If this function had a "$do_not_define" parameter, or the like, that would give it more flexibility. I suppose filing a bug report would be a step in the right direction to getting this idea considered.)