Is it possible? Sure. But, be aware that what you are now looking for is not the easiest thing to achieve. What you are looking for is the proverbial unicorn as they are called here; a single, bisexual female who is interested in and willing to be part of a triad of one sort or another.
Probably a simple majority of people have never heard of swinging, or something close to a majority. Of the people who have, the vast majority of them have never seriously considered it as an option for themselves. It's just that 'weird thing that those strange people do'. This presents obstacles to finding a woman among 'vanilla' (non-swingers) people (such as this worker at the restaurant you have mentioned). Not only does she need to be interested in the idea of dating you, but she needs to be comfortable with the idea of you being married and with your wife being ok with you seeing other women. Some vanilla people..in fact probably most if the threads I've read here are any measure..are far more offended by the idea of a spouse playing with permission than they are of a spouse cheating. Toss in that you'd like her to be bisexual too. Now toss in that such women among swingers are in VERY high demand, which means that any such women have many, many opportunities available to them...and the potential pool of women who'd be interested is vanishingly small.
I'm not saying all this to bring you down. Rather, keep yourself realistic. It is highly unlikely you can go from 0-60 with this woman with your proposal. You're going to have to be patient, get to know her, let her meet your wife, do some light flirting, see if there's any spark between the two. This may take multiple times of visiting with her. Try to spend some time with her outside of her at work; casual like, not heavy. Perhaps include your wife. In my opinion, if you rush this your chances of success are worse than zero.
Now; to bring you up a bit. There are plenty of couples who have a girlfriend who is an active part of their sex lives. It's not at all unusual. Society doesn't speak of it because it's taboo, but it's real and while the % prevalence is not high, there are many thousands if not tens of thousands of couples who have a girlfriend. It may take time for you to find such a woman. This woman you speak of at the restaurant is just one person. It may take 10, 20, 30 tries before you find the right fit. It can be done though, and the only way to make it happen is to try.
Now, you've spoken much of what your needs are, and a slight bit about that of your wife (this notional girlfriend occasionally having sex with your wife). Far, far, far more important in this scenario is the needs of the girlfriend. Those must be paramount. Otherwise, this notional girlfriend is just a masturbatory tool you use. What's in it for her? What are her needs? Why would this be a good scenario for her? Why would an attractive, single, bisexual woman want to be in such a scenario when she can find other singles to fulfill her sexual (and emotional to boot!) needs?
There are women for whom this scenario can work very well. If they are truly bisexual, then being a girlfriend to two people who are married can be great; they get to have sex with both genders and not worry about problems of cheating, or being restrained to sex with one gender and always thinking about what they are missing with the other. That's one aspect. Another possibility is the single mom type. They have one or more kids. Finding a man to fulfill their sexual needs can be difficult. They don't have time, energy, etc. or the type of men interested seem limited. Having a married guy they can have sex with from time to time could be very useful; no relationship entanglements, not as much time and energy investment, and her sexual needs get fulfilled. Anyway, you get the idea. You need to strongly consider the needs and wants of the notional girlfriend.