how to add data to the json data to the combo box in sapui5 ?

use this method in the init method of your  controller.

var mData = {

"combotype":"ComboText",

"rajendra" : [{

"Name" :"Jan",

"Value":"1"

},

{

"Name" :"Feb",

"Value":"2"

},

{

"Name" :"March",

"Value":"3"

}]

};

var oModel = new sap.ui.model.json.JSONModel();

oModel.setData(mData);

this.getView().setModel(oModel);

and your xml view should contain 

<ComboBox   class ="Combo_box1" items="{/rajendra}" >

<items>

<core:Item key="{Name}" text="{Value}" />

</items>

</ComboBox>