Adding data to $post variable from a plugin

[4x4 Mecca]

I'm trying to expose more data to postbit_legacy inside of the $post variable. I thought I could do this from the fetch_postinfo call, putting the following code
$postcache["$postid"]['mynewvariable'] = $mynewvariable;
But it doesn't work. I'm obviously not understanding the $post template system. Where can I add information to this from a plugin?

Thanks!

Got it! I put it in show_thread_postbit instead