I think you could create the geopoints in desktop GIS software, like QGIS, by creating or importing polylines for your highways, then processing them to divide the line into a series of points with metadata calculated as the distance from your zero point. Then you would have your set of points with KP values that could be selected from map, selecting the closest to your enumerator.
If you don't wish to select from map and rather just grab a geopoint, you may have to post process these coordinates to the closest KP for your highway data, or do an in form calculate that compares the lat-long to a choice list with many highway KP/lat-long values to striking the closest coordinate match and return the KP