I don't remember checking specifically on a 7D, but every other Canon camera I've looked at, you can't set EC in manual mode....for the simple reason that the control you use to set EC, is the same control you use to set your aperture in manual mode.
On top of that, it doesn't make any sense to have EC while in manual mode. All EC does, is adjust the exposure settings, relative to the metered value. But in manual, there is no 'set' metered value...the 'needle' is not locked. If you want more or less exposure, you just chance one of the three variables.
This is why it's perplexing to me that Nikon does allow for EC in manual mode. All it does, is shift the scale so that when you get to zero, it's not actually zero. I just about time I teach my DSLR class, there is at least one student with a Nikon who has inadvertently changed their EC and had no idea. So all their practice shots of getting to zero, come out too bright or too dark.