Also check what Lord Xia said.
When you bring the pioneer inside the inner walls of the Imperium, start checking the tooltip for the 'settle' button - if your pioneer is standing on fertile ground but can't build a city because there is another one too close to that spot, the tooltip should say something about the site being too close to another city rather than the site being ineligible due to lack of food. Then look for the closest city to that spot, take it over, and raze it, then build your new city on the ruins of the Imperium. Generally, any city built adjacent to the borders of the wildland is a candidate for causing this particular problem. If you're lucky, only one city will have to be raised. If you aren't lucky, there might be two or three cities that you need to get rid of in order to clear the wildlands, and some of them may already be fairly well developed or have special structures in them.