No, you just pretend that Hagar wasn't a slave before and after sleeping with Abram.
Why does the "before" matter? Think about it: Just as a minor cannot "consent" to sex, a slave is in no better situation to "consent" to--or deny--her master's commands for sex. And in this case, the command didn't come from her husband, Abram; it came from her mistress (female word for "master"), Sarai (Sarai is twice referenced as "mistress"--Gen. 16:4,8).
Why does the "after" matter? Because it shows she didn't become a "transformed" person--from slave to wifely status. Gen. 16:6,8,9; 21:11; 25:12 all are still referencing her as either a "slave" (twice in 21:11), servant or one who was told by the Angel to submit to her mistress (female word for "master"). By Gen. 25, Abraham is married to Keturah with no mention of Hagar (25:1) and is then buried with Sarah (25:10).
I say to settle this matter, let's call the 5 key witnesses to the stand, and hear what they have to say:
Q Hagar, after Sarai gave you to Abram and Ishmael was conceived, did you still acknowledge Sarai as your "mistress" in your conversation with the Angel of the Lord? [female master]
A Yes. (Gen. 16:8)
Q Sarai, when you were in your early nineties when Isaac was a toddler, how did you characterize Hagar?
A I told Abraham, Get rid of that slave woman and her son, for that slave woman's son will never share in the inheritance with my son, Isaac. (Gen. 21:10)
Q Abraham, after Sarah gave you Hagar and you slept with her, how did you characterize Hagar?
A I told Sarah, as mistress (master) of her servant, Your servant is in your hands. Do with her whatever you think best. (Gen. 16:6)
Q When Sarah began to mistreat her servant, Hagar, did you intervene like what we might expect a husband to do?
A No. Hagar was Sarah's servant.
Q Angel of the Lord, when you called to Hagar after she conceived Ishmael, how did you reference her?
A Servant of Sarai (Gen. 16:8)
Q And when you conversed with Hagar, did you, Angel of the Lord, acknowledge that she was released from her servant role to Sarai?
A No. In fact, I told her Go back to your mistress and submit to her. (Gen. 16:9)
Q Moses, since you wrote Genesis, how did you identify Hagar in her last reference of that book? Did you link her to Abraham?
A No. I identified her as "Sarah's maidservant" (Gen. 25:12).
Q So in that same passage, you link Ishmael to Abraham, but you link Hagar only to Sarah?
Q Yes.
I did nothing of the sort. I only pointed out that the Hebrew word for "wife" when referring to Hagar is the same word used for "Wife" when referring to Sarai.
Are you denying that fact?