You can buy a 7 speed cassette. You might find a 7 sp with the right gearing would match your 6 speed free hub. You could set or leave the rear derailleur to not use the inner most gear. As your bike has down tube shifters and 6 speed its probably not indexed just friction so no issue. To use a 7 speed cassette on my kicker trainer I had to use a number of cassette spacers to move the cassette out on the hub.
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