You cannot unless you find the terrain shader in the unity contents folder, and change it to support toon shading. Your only other real option would be to use an image effect such as edge detect.

This might be pretty old but I figured it out

Shader "Hidden/TerrainEngine/Splatmap/Lightmap-FirstPass" {
Properties {
    _Control ("Control (RGBA)", 2D) = "red" {}
    _Splat3 ("Layer 3 (A)", 2D) = "white" {}
    _Splat2 ("Layer 2 (B)", 2D) = "white" {}
    _Splat1 ("Layer 1 (G)", 2D) = "white" {}
    _Splat0 ("Layer 0 (R)", 2D) = "white" {}
    // used in fallback on old cards
    _MainTex ("BaseMap (RGB)", 2D) = "white" {}
    _OutlineColor ("Outline Color", Color) = (0,0,0,1)
    _Outline ("Outline width", Range (.002, 0.03)) = .003
    _Color ("Main Color", Color) = (1,1,1,1)
    _ToonShade ("ToonShader Cubemap(RGB)", CUBE) = "" { Texgen CubeNormal }
}

    CGINCLUDE
    #include "UnityCG.cginc"

    struct appdata {
       float4 vertex : POSITION;
       float3 normal : NORMAL;
    };

    struct v2f {
       float4 pos : POSITION;
       float4 color : COLOR;
    };

    uniform float _Outline;
    uniform float4 _OutlineColor;

    v2f vert(appdata v) {
       v2f o;
       o.pos = mul(UNITY_MATRIX_MVP, v.vertex);

       float3 norm   = mul ((float3x3)UNITY_MATRIX_IT_MV, v.normal);
       float2 offset = TransformViewToProjection(norm.xy);

       o.pos.xy += offset * o.pos.z * _Outline;
       o.color = _OutlineColor;
       return o;
    }
    ENDCG

SubShader {
    Tags {
       "SplatCount" = "4"
       "Queue" = "Geometry-100"
       "RenderType" = "Opaque"
    }
       Pass {
         Name "OUTLINE"
         Tags { "LightMode" = "Always" }
         Cull Front
         ZWrite On
         ColorMask RGB
         Blend SrcAlpha OneMinusSrcAlpha

         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         half4 frag(v2f i) :COLOR { return i.color; }
         ENDCG
       }
CGPROGRAM
#pragma surface surf Lambert
struct Input {
    float2 uv_Control : TEXCOORD0;
    float2 uv_Splat0 : TEXCOORD1;
    float2 uv_Splat1 : TEXCOORD2;
    float2 uv_Splat2 : TEXCOORD3;
    float2 uv_Splat3 : TEXCOORD4;
};

sampler2D _Control;
sampler2D _Splat0,_Splat1,_Splat2,_Splat3;

void surf (Input IN, inout SurfaceOutput o) {
    fixed4 splat_control = tex2D (_Control, IN.uv_Control);
    fixed3 col;
    col  = splat_control.r * tex2D (_Splat0, IN.uv_Splat0).rgb;
    col += splat_control.g * tex2D (_Splat1, IN.uv_Splat1).rgb;
    col += splat_control.b * tex2D (_Splat2, IN.uv_Splat2).rgb;
    col += splat_control.a * tex2D (_Splat3, IN.uv_Splat3).rgb;
    o.Albedo = col;
    o.Alpha = 0.0;
}
ENDCG  
}

       SubShader {
       Tags { "RenderType"="Opaque" }
       UsePass "Toon/Basic/BASE"
       Pass {
         Name "OUTLINE"
         Tags { "LightMode" = "Always" }
         Cull Front
         ZWrite On
         ColorMask RGB
         Blend SrcAlpha OneMinusSrcAlpha

         CGPROGRAM
// Upgrade NOTE: excluded shader from OpenGL ES 2.0 because it does not contain a surface program or both vertex and fragment programs.
#pragma exclude_renderers gles
         #pragma vertex vert
         #pragma exclude_renderers shaderonly
         ENDCG
         SetTexture [_MainTex] { combine primary }
       }
    }


// Fallback to Diffuse
Fallback "Diffuse"
}

This is a slightly late response but actually, you can

change the first line of your toon shader code to the same name that the default terrain shader works and unity will use your terrain shader (your toon shader) instead of the default one. i.e.: the first line of your toon terrain shader should be:

"Shader "Hidden/TerrainEngine/Splatmap/Lightmap-FirstPass"