Hi, can't help ya with making custom tracks with Blender. Will have talk with others here in the forum.
About the Lawnmower derby pit, be warned where the straw bales outer wall is, it actually an invisible barrier wall with the straw bales place in the same spot. Found this out when I added some exploding straw bales to the Lawnmower derby pit for the Cops and Robbers mod. So you can't make this pit bigger or smaller. The invisible wall will still be there.
I been able to use the Stuntfest 3D editor to add and move objects on the stock tracks, but there tricks to be done to get Wreckfest tracks to load in Stuntfest 3D editor. And some tracks the objects don't line up with track surface when loaded into Stuntfest.
Here link to my Stuntfest 3D editor thread:Using Stuntfest editor as a Wreckfest 3D object editor.