yes, it is possible to shorten the shock with spacer between main cylinder cap and damper piston, but it is a bad idea. for example you have 241*76 shock. you wan to make it 222mm length. you insert 19 mm spacer and now you got 222mm shock with 57mm (76-19) of shaft travel. but 222mm shock must have 63 mm shaft travel and all damping circuits in the shock are designed for 76mm shaft. so you ruin your suspension kinematics, damping performance and decrease shock life. does that worth it?