Your units are not consistent. First of all, a kg is mass not a force, whichis Newtons (N). 1 kg = 9.8N
so 7 kg cm/cm is 68.6 N-cm/cm or 68.6 N-m/m. Now a KJ is 1000 N-m and a KJ/m2 is thus 1000N/m. Your first value is per thickness and the second per area so you need to know width of specimen to compute area; otherwise you cannot convert. Some impact strengths are reported per length and others per area (like Charpy vs. Izod) and test methods are different so you need to be careful in conversion.