You cannot, but, Acrobat/Reader will output it to print all on Letter, as will most other pdf apps, but they will all cause large margins on one axis do to the fact that it won't make the image smaller in one direction, it keeps the aspect ratio so Legal-Letter for instance has to reduce the long edge to 78%, and it will then do the same to the short edge (otherwise fonts can become unreadable, pictures distort etc etc)
I don't think Xerox would even consider adding the ability to change the output size of a mixed job digitally due to the fact from an Engineering standpoint, the page size makes no difference digitally, and all the applications typically have the option to change it when printed.