Cuckolding as explained by you in your comments is right. However this is the next phase. The first phase is to get your wife to agree to having a threesome - with another man. This may not be easy. You will have to gauze to what extent your wife will go to fulfill your fantasies.
The bottom line is that she must be excited to sleep with another man. This is only possible through communication. What is her re-action when she watches threesome porn with you? Does she show any emotions. One give-away is the sex that follows such porn viewing. The woman will be extra wet and raring to go. Even if nothing is said, you can be sure she is playing out the scene in her mind.
Get her to talk about men (actors) she fantasizes about. When you are making love to her, get her to imagine and name the other man (it could be a fictitious name). Once you have been able to convince her (never, never easy), then the next part, would be for you to get the man, you think is trustworthy and free of any STDs' etc.
In Metro cities, there are masseurs who will give the woman a massage - highly erotic and then with the concurrence of both will procced to sex. You can either watch or leave them alone. Book a hotel for this purpose, so that your identity is not revealed. You cannot trust anyone these days. Insist on use of condoms at all times.
Finally, back off the moment you sense that she is not willing. If you mess it up now, you will not be able to broach the topic later.