That requires customizing the Workspace source which is shipped with the LiveCycle ES SDK.
There might be another way.
The buttons on the chrome show because you have a submit button on you form.
You could add a regular button on your form with some script that show a message box, and then calls the existing submit button to do the submission.
On the regular button you would have something like (from memory);
xfa.host.messageBox("Please add an attachment");
This would show a message box and then call the click event of a submit button. Just make sure to hide the submit button and Workspace will take car of hiding the regular button.
Thanks Jasmin ~ How will the button know that I only want to show the pop-up box if they click on "Forward to RP" route/button? The other routes cannot have the pop-up.