In the BMC the additional de facto pārājikas are discussed at the very end of the pārājika chapter.
The application of the term ‘pārājika’ to these extra actions, though commentarial, is undoubtedly correct, for a lifetime ordination ban on one who commits any of them is given in the Vinaya Pitaka.
The particular one that you refer to is called ‘communion by theft’ (theyyasamvāsa), though I believe the interpretation you’ve been taught is overly strict. As I understand it, a layman who went to a fancy dress party in bhikkhus’ robes wouldn’t be committing theyyasamvāsa unless it was his intention to make the other partygoers believe that he really was a bhikkhu.
Here’s the relevant passage:
48. The one living in the community by theft
At that time there was a certain man from a good family who had been brought up in comfort, but whose entire family had died. He thought, “I’ve been brought up in comfort and I’m not able to make any money. How can I live happily without exhausting myself?” And it occurred to him, “These Sakyan ascetics have pleasant habits and a happy life. After eating nice food, they sleep in beds sheltered from the wind. Perhaps I should just get myself a bowl and robes, shave off my hair and beard, put on ocher robes, and then go to the monastery and live with the monks?” And he did just that.
When he came to the monastery, he bowed down to the monks. The monks asked him, “How many rains do you have?”
He said, “What does ʻHow many rains’ mean?”
“Who’s your preceptor?”
“What’s a preceptor?”
The monks said to Venerable Upāli, “Upāli, please examine this person.”
That man then told Upāli what had happened. Upāli told the monks, who in turn told the Buddha. He said,
“Anyone living in the community by theft should not be given the full ordination. If it has been given, he should be expelled.