To my knowledge, such forum plugins do unfortunately not exist for SMF.
But I would doubt the use of it. Yes instant messaging is quicker, but who is going to use it often?
Most people I desire to directly chat with are either on my FB, Steam or on Skype.
With an instant messenger the issue can arise when somebody simply wants to browse the forum, suddenly gets contacted by someone who wants to speak to him/her while they are not in the mood for a chat or don't have the time to get involved into a conversation. If they simply ignore the message, it can lead to a fight of blaming one from ignoring another. With PM you don't have this pressure.
Hotmail/Outlook did this not very long ago. They hooked MSN to your email inbox.
When I just wanted to check my email before going to bed, 3 people started talking to me out of nowhere!
It's now disabled though.