Can I sue my husband’s girlfriend for conversion?

First of all, you can get a divorce whehter or not your husband had an affair. Hower, marital misconduct is important insofar as o teh amount of alimony that may be granted. Additionally, you may be able to sue your his mistress if certain legal requirements are met. "Alienation of Affection" is the legal action based on a thrid party's willful and malicious interference with marriage relations. These actions are often brought along with "Criminal Conversations" actions. 
For a plaintiff spouse to recover for Alienation of Affection, it must be demonstrated that: (1) the spouse's were happily married, and genuine love and affection existed between them; (2) such love and affection was alienated and destroyed; and (3) the wrongful and malicious acts of the defendant brought about the loss of such love and affection. The exclusive right of sexual intercourse is not the right that is protected in such a suit, but rather it is the actual interference with the right to affection between spouses that's protected. Criminal Conversation is a claim for adultery; it protects the fundamental right of exclusive sexual intercourse between husband and wife. To prove such a cause of action, the plaintiff must be, or have been at the time, lawfully married and their spouse had sexual intercourse with the defendant without the consent of the plaintiff during the marriage.