in the server, you have the setting for main surface texture (will use Grass17.bmp as an example since its used on zoric). for the overlays, the server will automatically look for Grass17ly01.bmp, upto around ly10 or so. you don't really have to set up anything besides making the textures and making sure thier in the folder with the main surface texture.
the overlays are laid over roads to add the grass sides and used in the Lay Tile menu. you need to take the main surface texture, copy it several times to make the ly# files, and edit them to add in blue 255 to let the roads show threw where you want.
i.e. for zoric, i have:
ly1 as a turn. Layer 1 gets rotate in-game 4 times
ly2 as the horizontal road grass sides.
ly3 as the vertical road grass sides.
ly4 is the 4-way intersection.
ly5 is the diagonal road piece. Layer 5 gets rotated in-game 4 times
ly6 is a T-intersection piece.
could easily use whatever setup you want.
also don't have to use overlays at all, roads would just appear as roads with no grass siding