I want to add some Velocity controll to my CharacterController

Below I will post my Playercontroller script. In there I have a velocity variable that will controll the players movement.
I want to be able to make plattforms and other such things that will make my player fly up in the air when they interact and such.

To test the mechanics I am trying to add velocity when pressing jump, however; this is not taking effect.
The reason is because the velocity is reset to my "Horizontal / Vertical" * movementSpeed. How would I best go about on fixing this?

Sorry if I am unclear; I have a terrible headache and my main language is Swedish.

 using UnityEngine;
 using System.Collections;
 
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPersonController : MonoBehaviour {
     // Public variables
     public float movementSpeed = 10.0f;
     public float jumpSpeed = 10.0f;
     public float mouseSensitivity = 1.0f;
     public float maxHeadTilt = 75.0f;
 
     // Private Variables
     private Vector3 velocity = new Vector3(0,0,0);
     private float verticalRotation = 0.0f;
     private Vector3 movement = new Vector3(0,0,0);
 
     // Private Objects
     private CharacterController cc = null;
     private Camera _camera = null;
 
     // Use this for initialization
     void Start () {
         cc = GetComponent<CharacterController> ();
         _camera = GetComponentInChildren<Camera> ();
     }
     
     // Update is called once per frame
     void Update () {
 
         // Velocity
         velocity.x = Input.GetAxis ("Horizontal") * movementSpeed;
         velocity.y += Physics.gravity.y * Time.deltaTime;
         velocity.z = Input.GetAxis ("Vertical") * movementSpeed;
 
         // Inputs
         if (Input.GetButtonDown ("Jump")) 
         {
             velocity.z = jumpSpeed;
             velocity.y = jumpSpeed;
         }
 
         // MouseLook
         transform.eulerAngles += new Vector3(0, Input.GetAxis("Mouse X") * mouseSensitivity, 0);
         verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
         verticalRotation = Mathf.Clamp (verticalRotation, -maxHeadTilt, maxHeadTilt);
         _camera.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);
 
         // Movement
         movement = new Vector3 (velocity.x, velocity.y, velocity.z);
         movement = transform.rotation * movement;
         cc.Move(movement * Time.deltaTime);
         Debug.Log (velocity);
     }
 }