Using the shortDescription property, you can add a tooltip to an existing action.

In the following example, we insert the open action into the tool bar and use the shortDescription property to define a tool tip:

Program: Adding a tooltip to the open action.
includeConfig=lucy/lspmap/TLcyLspMapAddOn_MapComponent.cfg

TLcyLspMapAddOn.openFileAction.toolBar.item = Open
TLcyLspMapAddOn.openFileAction.toolBar.shortDescription = Load data onto the map

The resulting tool tip is shown below.

open action with tooltip tool bar
Figure 1. The file opening action with a tool tip.

See How to remove an item from an action bar ? for detailed steps to retrieve the correct ID of the action and action bar, and to retrieve the path of the configuration file.