There are multiple ways to approach this. Mostly men try the direct way of asking their partners to try other men. Very few conservative qives/gf would be ready to try it this way. So instead of this you should try going into it in a different way, an indirect method of planting a seed in her mind.
Make her believe that you've let go of the thought of cuckoldry in real so she can get into the plan easily
Firstly you can start with roleplay. Get her to imagine that someone is watching you both having sex. Tell her to imagine it in this way that you both aren't a couple, you've never met each other, she's met you as a stranger for the first time and agreed for a One night stand. So now she's fucking a stranger.
Then introduce a name (Say Martin) or create this imaginary personality, that there's this tall nd hunk guy with big chest. You're that guy, so now, let's see how she'd like to get used by this guy.
Then slowly you can introduce a dildo in ur bedroom. You being a watcher, sit on the couch and ask her to play with the dildo as if this was Martin's cock.
In this way you're indirectly planting a seed in her mind of fucking other guys