For ballpark purposes, Nm and kgfm are the same thing with the decimal point moved. 10N is the equivalent force (which is what the little f means; kg is mass not force, N is force) of 1kg, because 10N ≈ 1kg. You can just move the decimal point to approximate: 100Nm ≈ 10kgfm.
It's actually 9.8 here on Earth, because the Newton is defined as the force of a 1kg mass accelerated by 1m/s². Earth's gravity averages out at about ~9.81N/kg (at sea level it accelerates a 1kg mass downwards by 9.81m/s², thus 1kg = 9.81N), so 1kgf = 9.81N. Well, to two decimal places.
If you can convert Nm to lbfft (again, lb is a mass, while lbf is the equivalent force of that mass when accelerated) you can convert kgfm to lbfft by moving the decimal point one to the left before you start.
You can also approximate to lbfft pretty easily. There's ~3.1ft in 1m and ~2.2lb in 1kg, giving about a 7:1 ratio of lbfft to kgfm - and therefore 0.7:1 for lbfft to Nm.
That'll give you 100Nm ≈ 10kgfm ≈ 70lbfft. It's actually about 5% out, so increase by about 5% when you're done (10% halved; 70/10 = 7, 7/2 = 3.5; 70+3.5 = 73.5lbft) and you're close enough for ballparking it all.