How about a simple yes or no?
Yes, Sarai gave her slavegirl to her husband as a surrogate wife (Gen. 16:3). Was the purpose for Abram to have wife #2? (No, are you crazy? Do you think Sarai wanted competition as either a wife or parent-sharing?) Was the purpose for Hagar to conceive & deliver a child surrogately? (Yes) Does a slaveholder have the absolute authority to command her slave to have sex with her husband? (I don't think so; but I know others read Scripture differently & disagree with me on that). Does nailing the word "wife" to the bedpost of a man about to have sex with his wife's servant girl make her his "wife?" (I don't think so; but I guess you conclude differently)