There are no fields for categories defined in the EXIF/IPTC/XMP standards and thus no microsock agency can fill categories automatically based on the image metadata.
The only way to set categories (and releases or other agency-specific metadata) and to have these fields automatically filled on the agency website later is to use software that imitates user actions on the agency website. However due to the complexity of the task such software is not free.
For setting titles, descriptions and keywords and uploading files there are few free software products around though, like
Xpiks or
StockSubmitter. They are NOT affecting JPEG quality when saving metadata. Also editing multiple images at once is possible.