Step 1: Select Options, Preferences and then Tools and Toolbars from the menu.
Step 2: Click on the button you want to add the tool to on the preview in the middle of the dialog.
Step 3: Choose the tool/function you want to add in the list called tools.
Step 4: Click on Add to selected button.
Optional step
Step 5: If the button already had a tool assigned to it you can make the new tool appear on the button by selecting it in the Selected button list and clicking on the arrow up button until that tool is at the top. Any other tools listed below will be available from the drop down box on that button on the toolbar.
Click on OK at the bottom of the dialog and you're done!