Place a flat bench in the Smith machine, with a stable wood block or similar object to place the balls of your feet on to provide a stretch. Sit at the end of the bench with your knees bent 90 degrees and your feet on the edge of the block. Unrack the bar and support it on your lower thighs.
Lower your heels to get a good stretch and raise them as high as you can for a good contraction. You are now doing seated calf raises without a seated calf machine.