You can achieve what you need in two basic manners:
1) add "fake" IDs on the server and add them to the TargetControls collection
2) use the RadToolTipManager client-side API and create the tooltips on the client when the item is hovered.
You can find demos which show the both approaches with a other RadControls below (no matter the controls are different, the tooltipifying logic is the same as in your case):
Please, examine teh demos, choose teh approach which better suits your scenario and use the sample code as a start point. In case you experience problems or you have additional questions, do not hesitate to contact us again.
the Telerik team