Quite recent game change (update adding pedestrian streets) introduced new setting in networks: VehicleCategory. If the road asset lane does not support emergency vehicle category TM:PE won't allow for them either.
In case of vehicle categories, TM:PE can only use existing settings from the network or add more restrictions to it, restrictions they cannot be lifted with the mod (it was way too complex to add), so no, you cannot change road lane settings in a way that allows new vehicle category even if it haven't supported it.

Keep in mind the game has editor, I don't see any issue with loading existing network asset (one you mentioned) and changing its settings, then saving as a new network (with slightly different name). You don't need to model mesh in Blender or prepare textures. Just load existing and change its internal settings.