How to create tappered thread in OpenSCAD?
If your math and OpenSCAD skills are superior to mine, you may be able to make use of the OpenSCAD Metric Nut, Bolt &amp; Threads Library located here:
OpenSCAD Metric Nut, Bolt &amp; Threads Library

It uses various means to generate polygons about a radius and includes the formulae for partial revolutions. It is presumed in the design that the center of rotation for the generated polygons is constant. I looked over the code for outside thread and could easily determine the radius references.

With proper coding, you could generate a variable radius based on the height of the cylinder at a specific point and achieve the tapered effect you require. 

I expect that you'd have to reduce your desired radius by a fraction, say 0.05 mm in order to embed the thread forming polygons within your tapered cylinder.

If you aren't a strong coder, disregard this answer.