So what’s really going on is this.
The notes in the 4/4 staff are really triplets because the measures actually only have 3 beats (to equal the length of the “real” 6/8. (You would use some other ratio of tuplet if you were trying to do something like 5/4 against 4/4 or something).
The first measure of the 4/4 section will be a pickup bar (Shift-M, 4/4,3) of 3 beats. This is just to get the symbol.
In the second bar, you create a time change to 6/8, and then hide it. So it looks like it’s in 4/4 still, but it’s not, it’s really in 6/8, again, to make it match up.
Then hide all the triplets and boom, it looks right, and plays back right.
It is a bit faffy and there certainly could be dedicated functionality to support this though.