I suspect if you have a patch of milkweed growing there now, chances are it will be right back same time, same place, next year. If you like it, it's for sure going to like you. The thing about milkweed is that it chooses its home most of the time. Sounds to me like it's already established.
Good for you!
In my experience it's a little difficult to transplant, but if you fall/winter sow some of its seeds you might add to your collection. Actually they'll probably sow themselves.
It's good to hear of your experience, thanks!
