No. If you open a mail merge document that you created with the Word desktop application, Word Web App will preserve all of the mail merge settings in it, but you can’t perform a mail merge or edit any mail merge settings.
If you have the Word desktop application, use the Open in Word command to open the document and perform a mail merge there.
Word Web App works seamlessly with Word and other Office desktop programs, and we update it based on input like yours. For the latest feature announcements, visit the Office Web Apps blog.