Kathy,
I did something similar for rainfall contours using ArcMap and Spatial Analyst. I documented it here.
https://www.contracosta.ca.gov/DocumentCenter/View/3914/Mean-Seasonal-Precipitation-Raster-From-Dwg-...
It was a while ago, but after looking at the document, I now recall I made a raster from the manually created (digitized) contours, from the raster generated finer contours, checked the accuracy of those finer (smaller interval) contours. Then I manually interpolated between the initial manual contours where needed by creating lines between the initial contours and then drawing new contours by clicking the midpoints of those lines. Tedious, but it worked.
Possibly what you need is somewhere in that process. ArcPro may have a simpler tool.
Mark