Is it permissible for an engaged couple to address each other using terms of endearment, such as "Bassoum" and "Hassoun," instead of their original names?

There is no harm in shortening a name or adding letters to it, as long as it does not involve belittlement or mockery. It was reported that the Prophet, peace and blessings be upon him, called Aisha, may Allah be pleased with her, "Ya Aish," and Anjasha, "Ya Anjash." This is prohibited if it becomes a form of taunting with nicknames and belittlement, based on Allah's saying: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers."