Assuming those shifters have a friction function, set them on that and they'll work. You'll just have to sense when the chain moves to the next gear. There are a number of riders who prefer friction to indexed click shifting systems. My personal bikes are half and half.
10-speed shifters won't index with 7 rear cogs because the 7-speed cogs are spaced farther apart than 10-speed cogs are. Consequently, 7-speed required more cable pull to "click" into place.
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