Is it permissible for a Muslim woman to be a ملك يمين (right-hand possession/concubine)?

The default state for humans is freedom. Slavery is only an exception. It is not permissible to enslave a free person, even if they consent, because that would nullify Allah’s right. A slave enters a person's ownership through the enslavement of captives and war spoils from disbelieving enemies, or when the child of a female slave inherits their mother's slavery, or through purchase from someone who rightfully owns them in a manner recognized by Islamic law.

Therefore, it is not permissible to enslave a Muslim, unless they were enslaved during their time of disbelief and then embraced Islam, or if their mother was a slave and they inherited slavery from her. It is not permissible to enslave a woman who was not enslaved in a war against disbelievers, or if slavery was not legally established upon her, even if she consents to it.