I've managed to get my Sims to interact with the Grim Reaper on occasion. I'm not really sure what the formula is. Thinking back, at least one was brave, one had high charisma, one was neither brave nor charismatic. After Grim took one of my Sims, I had her ghost flirt with him, but he rejected a first kiss and then disappeared. It seems like no one can interact with Grimmy unless he's planning on hanging out for a while. So, if he makes a drink at the juice bar or sits down to read or play video games, there's your chance.