If we follow Angular EventEmitter API: https://angular.io/docs/ts/latest/api/core/index/EventEmitter-class.html

1. change the EventEmmiter declaration to use : EventEmitter
2. To send the data you have to use the emmit method.

Your code should be like this:

@Output() cellClick: EventEmitter<any> = new EventEmitter();
private _cellClicked(data: any){
      let emitData: any = {
        colId: data.column.colId,
        rowId: data.node.id,
        item:{}
      };
      if (emitWithdata){
          emitData.item=data;
          this.cellClick.emit(emitData); // Here  i need to send my data also, and it need to append to the value Data.item=data not just the Data object.
      } 
}

To send data back to event handler, you can write like this:

this.cellClick.emit({data : emitData, orderData: someData});

This is how you do it as of Angular 11. I'm sure older versions are somewhat similar. Also, you can learn more details by reading the documentation on Event Binding.

You can definitely use an any object declared on the fly, however this example will show how to do it using a strongly typed object.

A strongly typed object, declared somewhere in your application:

export interface MyCustomObject {
    rowId: number;
    columnId: number;
    item: any;
}

Child markup:

<div (click)="onCellClicked()"></div>

Child component's TS file:

import { MyCustomObject } from './some-file.models';

@Component({
    selector: 'child-component',
    templateUrl: './child-component.component.html',
    styleUrls: ['./child-component.component.less']
})
export class ChildComponent implements OnInit {

    //whatever name you assign this Output variable,
    //is the event name you'll bind to on the parent markup
    @Output() cellClicked = new EventEmitter<MyCustomObject>();
    
    constructor() { }
    
    public onCellClicked(): void {
        const obj: MyCustomObject = {
            rowId: 5;
            columnId: 24;
            item: {
                foo: 'abc'
            };
        }
        
        this.cellClicked.emit(obj);
    }
}

Parent markup file which makes use of Child component:

<!-- $event param will hold the passed value -->
<!-- also notice the use of (cellClicked) as defined by the childs Output variable -->
<child-component (cellClicked)="onChildCellClicked($event)"></child-component>

Parent TS file:

import { MyCustomObject } from './some-file.models';

@Component({
    selector: 'parent-component',
    templateUrl: './parent-component.component.html',
    styleUrls: ['./parent-component.component.less']
})
export class ParentComponent implements OnInit {

    constructor() { }
    
    public onChildCellClicked(event: MyCustomObject): void {
        //do something on the parent with the data
        console.log('parent cell click event: ', event);
    }
}