I believe they refer to it as freeze concentration. I don't think it is illegal in most places but it really depends on where you live.
Distillation using a still is illegal in most places but freeze concentration not so much from what I understand.
I placed a 1/2 gallon plastic apple juice bottle in the freezer, left it for a couple of days. To thaw I simply placed upside down over the quart jar. When it was full I placed over the other jar. It is still aging in the pantry on some oak cubes.