Batch add an 8.5 x 11 canvas to both horizontal and vertical images

[winterludes]

I'm trying to quickly get some rough prints done at Printique, but they don't have any options to "print as the image is currently sized, they crop the image no matter what.  So my only option is to add a canvas to every image so they're all 8.5 x 11 and then print with no border.  

But I can't figure out how to batch this.  I've got about 500 imges so I can't do it manually.  But there must be a way to do it or some other app that would do it?  Or a way to do it in Lightroom?  

Thanks for any advice!

[Stephen_A_Marsh]

What width and height of the document in pixels?

Are all your images the same pixel width and height?

Can you provide a before and after visual example?