Can Altium Designer 17 Generate Missing Paste Mask Layers?
With the appropriate (eg soldermask file) go to the Tables/Apertures, where you should be able to change the aperture dimensions as necessary.  

Do this on a copy of the original of course.  

You can also do this in the final gerber file as well, the standard is available at the www.ucamco.com (the_gerber_file_format_specification.pdf)

The hitch is that if there are paste mask patterns that are not simply smaller versions of pads you may have to create some new ones and fit them in by hand (often these are thermal pads, etc.)

You should then double check the resulting gerber file either with camtastic or with GCPrevue or some other such program that can stack the soldermask and past mask layers in different colors so you can inspect visually.