Well, this is not a support forum for 3DSimEd or track creation, and despite i'm not the more skilled 3DSE user around here I'll tell you what i'd do:
Select the Erase geometry function (EDIT tab), select all the materials except the one used by these tire walls, and delete them. Now you only should have tirewalls on your screen. Delete all of them except one, exporting it as a rFactor .gmt
Open your track in 3DSimEd, go to PRIMITIVES tab and use Add Object function to load the .gmt you exported before. Your single tirewall will be loaded, just place it wherever you want it to be.
There're other ways to do it, but this is just one of them. For more info check track modelling dedicated forums ;)