Combining .exr files? Is it possible to composite multiple .exr files into a single .exr file? 

For example, if one .exr file contains several passes and another contains only one pass could they be combined to contained all in one file?
Yes you can load several EXRs on the compositor and save all of their channels into individual layers on an OpenEXR multilayer.

Add a file output node, set it to OpenEXR muyltilayer and than add as many inputs as you need. 



You can rename the channels as needed by double clicking on the name:



For more details see this answers:

https://blender.stackexchange.com/a/18406/1853

How can I render all render passes to files in Cycles?