If the tires are still good enough to turn in then maybe buy a set of 18" winter rims and put what you want on them, winter, all season, whatever. Drive them till the lease is done, turn it in with the worn but still legal tires and make sure the next lease will take the winter rims and tires you bought
PS Read your lease carefully..I was about to go over KM on my leaser and wanted to know how much it was going to cost me...the spot where its supposed to say 5 or 10 cents per km says N/A.......booonus...somebody messed up when typing it