It is very possible... I think very likely as a matter of fact. Has your pond had a flood event where another body of water touched yours through an outflow pipe? Is your pond fed by a stream that has a pond further upstream of it? The crappies could have washed down stream from the other pond into yours... These are all possible ways the crappie(s) could have found their way into your pond? An unknown unwanted fisherman could have dumped a few in your pond as well... Some think birds are good fish spreaders as well? It may simply be one lone crappie. What size pond do you own? Often times crappie struggle to pull off a successful spawn in a small pond, so there may be no issue as the original stockers may die off in few more years.