Can a form make itself modal in mid-flight?

I have a modeless form which has a grid on it, with one column as a checkbox.

The new specification is that if the user clicks on any of these checkboxes, they must visit (and change) a new textbox (not in the grid) before exiting the form.

Can I make the form modal when the user changes a checkbox? If not, and it remains modeless, they can cheerfully go anywhere else in the app, never to revisit this open form if they so choose.

Any suggestions welcomed. Thanks