Just do it like in plain C. But remember, pointer are only valid within the scope of the kernel.
An example:
__kernel void mykernel(__global int *A, __global int *B, __global int *C)
{
int id = get_global_id(0);
__global int * p;
if(A[id] > 10){
p = A;
}else{
p = B;
}
C[id] = p[id];
}