What kind of conditions are you talking about when you refer to confined spaces. I hope that you are not expecting your employees to go down into manholes, particularly sanitary sewer manholes without proper training and equipment. These are considered to be hazardous because of traffic concerns at the surface and because of gasses within the structure which can and often do displace breathable air. A man descending into a ss manhole with significant amounts of methane wouldn't know it until it is too late. Once down in the hole, he would not be able to get back out.
In some, if not all jurisdictions, requiring employees to work under these conditions also requires the employer to provide appropriate training, which often means hazardous materials/conditions training from someone certified to provide it. That's a statutory issue.
Additional pay is an employment contract issue. It may be statutory in some locations, I don't really know and wouldn't be surprised either way. If your employees are union, there is probably a hazard pay provision somewhere in the contract.
If the confined spaces are such that they present no danger of trapped gasses, collapse, or other mishap which could result in the employee being trapped, hurt, or killed, there is probably little to be concerned about. But to be safe, find out what laws there are in the labor or health & safety codes in your state. Some level of mandatory safety training may be required. If so, that's on you.
The other alternative is to get the information you need without having anyone enter the space. In the case of manholes, in 30 years of surveying, I have never ventured down into a manhole that has already been in use, nor have I ever required anyone working for me to do so. I have prevented a couple of gung-ho knuckleheads working for me from doing so over the years. There is no need to have a survey crewman enter a manhole. There are methods of attaining measure down info accurately enough for most applications with commonly carried survey equipment, and there is specialized equipment available if the standard methods of measurement are not precise enough.
If you were to fire a crew member for refusing to enter a legally defined hazardous situation without the required training and equipment, it could be quite costly to you. Research your situation. If the confined spaces are something rather unique to a particular market you serve, decide how important holding your portion of tha market is to you, whether paying skilled workers familiar with it something extra for the hazardous portions is worth it to keep those workers in your organization, and whether you have the right workers in place now.