Can I add text to the back of my predesigned template?

Yes!  You just need to upload that text as an image.  The back of the templates will not support typing in text, but you can upload text as an image to the back of a ticket.  On the predesigned template, the image on the back of the ticket is on the body of the ticket and not the stub, and takes up a portion of that space only.

An easy way to do this is to save a word document as a pdf, then save the pdf as a jpeg or png.  You can also try taking a screenshot of the text on the word document and uploading that.  That's a great solution, as long as the screenshot is legible.