Not what you're looking for, but this is how I do it using this mod, which btw works also with v3.6
Adv. Editing Options for Mods and Admins
Essentially this mod, in addition to lots of other things enables you to define the date/time stamp of a post. (don't forget to check the box next to the new time stamp to make it effective)
As such, what do re your situation is create a post in a private forum with the time stamp between the dates of the two posts in the thread you want it inserted to, then after submitting the post, simply move the post to this thread.