On my laptop I have a copy of a Citroen training manual for that box. In response to your question, I have located diagrams showing drain plug, and dipstick, but the filler plug does not seem to be shown, although it is mentioned.. Perhaps you are meant to use the breather which is close to the oil cooler, on top of the box, but how accessible is the breather. Another possibility may be via the oil cooler.
The complete file is over 3MB, so rather large to attach to a post, but if you are interested and supply your email via PM, then I could send the manual via email.