There was a recent thread about installing a 7 speed cassette on a trainer (linked below) and the answer is yes. However, the next question is whether the frame has adequate spacing between the rear dropouts to fit on the trainer. Some older 7 speed bikes have 126mm rear spacing, and the trainer needs 130mm or more. You should measure the rear spacing to see if it will fit, or ask a bike shop to take a look. Steel frames can be realigned to provide more space, but it’s not without risk and should be done by an expert mechanic or frame builder since it involves bending the frame. That’s not a good idea if the frame is made of aluminum.