You could certainly select thrust reverse in the air if you tried, but it wasn't an approved or required technique. In any case, the Air Engineer would probably have hit you!
That well-known aged ex-TP Scots FI "Sooooo, co-pilot...." had one go into reverse in flight in the descent. The greater that thrust they applied, the greater was the rate of descent...until they worked out the cause. But much more exciting was when the RB211 flying test bed had the RB go into idle reverse in flight...full power on the 2 Conways and it still descended. There were other problems as well, but they finally got it sorted out at 1000ft and landed at Filton.