The ISS orbits at ~420 KM.
Epotential = mgh = 420,000m * 9.8 m/s^2 * mass ~= 4,200,000 m^2/s^2 * mass
To reach that altitude, ignoring air resistance, you'd need kinetic energy, provided by the formula:
Ekinetic = (mv^2)/2
Solving for v, we get v = sqrt(8,400,000 m^2/s^2) ~= 2.900 m/s ~= Mach 9
That's the minimum muzzle velocity you need to send a projectile up into the ISS's path (Add a few more mach numbers due to air resistance).
It's certainly easier than getting into orbit, a single-stage missile could do it without any trouble.
