I doubt you can. Your boss will tell you he can't pay you more than what's on the grid negotiated with the union. My wife works in a place with a union (and is a member of it), and there is such a grid. Nobody even thinks individual bargaining is a possibility.
If you are in a union shop, you can't get paid more than what's on the grid. If you want more, you need to resign and get somewhere else. But don't pretend you have a choice to do "individual bargaining".
Unions eliminate the individual bargaining power of people.
Except, of course, if we choose to use your definition, and call resigning and moving to another job "bargaining".
You not liking their choice doesn't mean there was no choice.
Just join the union, you'll get paid more anyways.
You don't.
If you were to come to my company and try to convince me to start a union, I'd say "thanks, but no thanks".
Just now, you are trying to convince me that I'd be retaining my individual bargaining power, if I accept to use a different definition.
Well, I was born in a Communist country, where they were fond of changing definitions. We were free to vote for whomever we wanted, as long as it was someone on the list presented to us. We had no say in who was on that list. Of course, we were free to not vote at all, right? Boys had to do mandatory military service. You know how they were called when they were conscripted? "Volunteers". That was the official name.
So, yes, I'm used to the tactic of changing definitions. But if you are serious about presenting an argument, you should no use this tactic.
Why would you want a less powerful version of the same thing? Even if you don't involve your union, you can still do what you're describing.
You've gained nothing unique by avoiding the union.