Adding custom fields to an images metadata dictionary

Is it possible to add custom dictionary fields to an images metadata?

The following statement

writeImageDataToSavedPhotosAlbum:metadata:completionBlock:

Allows images to be added with metadata.

Is this dictionary (the metadata dictionary) treated as a normal dictionary?

I mean, Is it possible to add custom fields to it.

Of course the original metadata dictionary needs to be copied, but once this is done can it be edited and used.

Also when would this be done? If an image is captured with the iphone and I want to add stuff to the metadata, will this be writing an image twice, or is the first captured images metadata available, modified and then the image is saved with the metadata?