I've not found any way to fool Unity and make it "think" a key was pressed, but you can create a function to substitute Input.GetAxis, for instance. Add the code below to the script where you read Input.GetAxis("axis"), and just remove the dot to transform the original function calls in InputGetAxis("axis"):

private var axisH: float = 0;
private var axisV: float = 0;

function InputGetAxis(axis: String): float {

    var v = Input.GetAxis(axis);
    if (Mathf.Abs(v) > 0.005) return v;
    if (axis=="Horizontal") return axisH;
    if (axis=="Vertical") return axisV;
}

function OnGUI(){

    axisV = axisH = 0;
    if (GUI.RepeatButton(Rect(50, 10, 40, 40), "W")) axisV = 1; 
    if (GUI.RepeatButton(Rect(50, 90, 40, 40), "S")) axisV = -1; 
    if (GUI.RepeatButton(Rect(10, 50, 40, 40), "A")) axisH = -1;
    if (GUI.RepeatButton(Rect(90, 50, 40, 40), "D")) axisH = 1;
}

Use this InputGetAxis like the conventional Input.GetAxis, like below:

dirH = InputGetAxis("Horizontal");
dirV = InputGetAxis("Vertical");
...

my god "Aldo Naletto" thank you so much, spent absolutely ages trying to get something on the lines of this working, <3

thanks, nice code. But I aks emulate rotation of mouse in de axis x in two button

var rotateSpeed = 20.0;

function Update () {
transform.Rotate(Vector3.down, Input.GetAxis("Horizontal") * rotateSpeed *   Time.deltaTime);