How to add new custom icon into existing toolbar?

Right now, if the user wants to add any custom action into the existing toolbar (print, search, filter), then it is not possible. We have option to use custom toolbar only rather than to add custom icon into toolbar

## Expected Behavior
We should allow the option to add any custom action (e.g Add) into the existing toolbar

## Current Behavior
Not able to add my custom action into the existing toolbar

## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | 4.11.0 |
| MUI-datatables | 3.2.0 |
| React | 16.8.6 |
| browser | chrome 95.0.4638.69 |

No source file or test is named. Start by locating the existing toolbar and the custom toolbar option, then trace how print, search, and filter actions are added. The work is done when a custom action such as Add can appear alongside those existing actions without requiring a fully custom toolbar.

Written by the indexing model from the issue text.