Indeed use at least a monopod and preferably a tripod with that combo!
It's slow, long and heavy which is an open invitation to severely shaken images.
Usually, teleconverters are designed for specific lenses. I believe the Sigma converters are optimised only for certain of their lenses. They may work on other brand lenses, but with less than optimal results.
Teleconverters are also not usually designed for zoomlenses (with exceptions like the Nikon 80-200 f/2.8 AF-S). You'd best check with a reputable dealer about it before making the purchase and ask to try it as the Sigma catalogue and website do NOT list the 170-500 as being compatible with their teleconverters http://www.sigmaphoto.com/html/pages/ConvComptChart.htm
At best you just loose your autofocus and/or aperture settings, at worst you'll damage one or more of the components of lens, TC or camera when mounting the TC.
You're probably better off getting 2 Canon 2x teleconverters and stacking those (possible, but costs even more light). This would give you a very slow 300-1200mm lens