Does steeping crystal malts contribute any fermentable or unfermentable sugars to your wort?
Yep, A crystal malt will contribute both fermentable and unfermentable sugars. The left over sugars that don't get fermented add an extra "sweetness" to your brew. It will also add some nice caramel type flavors.
Yes it will contribute both both fermentable and non-fermentable sugars to the wort. Much of the sugars extracted are non-fermentable and give the beer the caramel flavors and color.