Yah, sadly I don't think this command even exists. I've scanned several different sources that make reference to WoW Slash commands and /in isn't one of them. I figure it's all a rouse or he's got an Add-On we don't know about. Maybe their's a custom package that allows this.

Can you define your own custom slash commands with scripts? Could you just write a custom script that would do the multi-part dialog for you? I figure you can write a script to pause the dialog for you, could you?