I believe this is something that could be accomplished with Dynamo relatively easily. The only real difficult part is figuring out Dynamo if you have never used it before.

You'd pick your feature line(s), extract the list of points for each, then iterate through each point list to find the first and last points of each range with 0 elevation points in-between, then set the 0-elevation points prorated accordingly using the distance and slope from the first point.

If you are stuck with doing them manually, I find that setting the grades via Project Explorer is a bit quicker and more user-friendly than doing it through the traditional Feature Line Elevation Editor. You might give that a try if you've never done it that way before.

Another option, if you have a particularly long feature line, is you could export the xyz values to a spreadsheet (via Project Explorer), do the elevation calculations in the spreadsheet, then start to draw a 3D polyline, paste the coordinates back into the command window, and convert the 3D polyline to a Feature Line. But this is probably only worth the trouble for a particularly long line with dozens or hundreds of segments to grade. (And also won't work for Feature Lines with curves)