Well you need the specific activity of the enzyme to do so. One microkatal corresponds to the amount of pure protein required to form 1 micromole of product per second. So this amount could be expressed as a mass if you have the relationship between microkatal and mass of the enzyme (which is the specific activity).
As 1 µkatal is 1µmol per sec and as 1U is 1µmol per min, 1µkatal is 60 µmol per min (ie.60U). So the relationship between the 2 is actually : 1U = 0.0166 µkatal.