There is an old Minhag Ashkenaz to sing a piyut by R' Avigdor Kra called Echad Yachid Umeyuchad in honour of a chasan being called to the Torah. The piyut is an acrostic which spells out אביגדור קרא. For ק, the verse reads "קיני נעמן גם עובדיה רחב נעמה ורות המואביה נכנסו בדת תושיוה הללוקה." Of course, Ruth the Moabitess converted to Judaism, as we clearly see in the Scroll of Ruth. My question is where do we see the others here converting to Judaism?
1) Keni was one of Yisro's names. (See Rashi who brings the Mechilta about his many names and Rashi further mentions there that he converted; Shmos 18:1)
He is famous for exclaiming that now he knows that G-d is the greatest of any of the powers. (Shmos 18:11) The Midrash says that when Yisro said this, it was the final words G-d wanted to hear before deciding to give the Torah to Israel.
2) Na'aman refers to the general of Aram mentioned in II Kings 5.
He is famous for saying a similar statement to that of Yisro. (see verse 5:15)Verse 18 shows he realized idolatry was terribly false and asks forgiveness for that sin. There seems to be a machlokes (Gitin 57b vs. Mechilta Yisro) about the level of Naaman's conversion. Was he a "ger toshav" or was his conversion on par with or higher than Yisro's?
3) Ovadiah was the prophet of the Book of Ovadiah. He converted to Judaism from Edom. (see Rashi to Ovadiah 1:1 who brings Sanhedrin 39b.)
4) Rachav was the woman who hid Pinchas and Calev in the Book of Joshua ch.2. Tradition has it that she converted and married Joshua. (see Megillah 14b)
5) Na'amah refers to King Solomon's wife who was an Ammonitess who converted. She was the mother of King Rehoboam. She was considered to be very righteous. (see I Kings 14:21 and Babba Kama 38b.)
6) Ruth of course is Ruth the righteous convert, from Moab, the great-grandmother of King David who is the main character in the Book of Ruth.