Hiya CeesDonker.
Here's how I would do it.
If you just want a down and dirty white circle, not "blended" or anything fancy, you can easily do it with FSSC. Decide what you want the diameter of the circle to be. Then(1) in fssc create a simple 2-point taxiway that is as wide as the desired diameter of your circle. Next(2) select the taxiway end point and move it so that it coincides with the start point- so that they sit on top of each other. In the FSSC layout window this yields a taxiway that is displayed as a circle. You will use this displayed circular taxiway as a template for a circular polygon. Unfortunately, you can't use the taxiway so created in the sim because, since it has no actual length, it will not display in the sim. Next(3) create a polygon whose points follow the outline of the taxiway circle in the FSSC layout window- zoom in close so that it is accurate. Next(4) once you have created the polygon, delete the taxiway and you will be left with just circular polygon. Make sure that the compiled poly will be white by choosing white in the polygon properties window. Or you could use a texture that is whitish. Compile. Should work fine.
Cheers,
MR
PS I am using an older version of FSSC. I am not sure whether the newer version displays taxiways with circular ends.
PPS: I just checked. The newer version does not display taxiways with circular ends. I normally use Version 1.2.15 for this very reason. Many later airfields that I made required lots of circular polys for large B-29 parking pans.
If you need a copy of the older version of FSSC, 1.2.15 i think it is, there is one included in my scenery-making tutorial which is available at my website under the "Other Stuff" link. Maybe available here at SOH too.
For some reason I couldn't get the attached images to appear in the order I selected them. The pics below should be viewed 4-1-2-3 to follow along with the instructions. ;^)