In Windows Disk Management you can right click the volume you want to extend (eg C Drive) and choose extend. You can then allocate the available space to that partition and extend it.
Yeah I tried that - but even after formatting the un-allocated space and not assigning drive letter or anything, the option to extend C drive is greyed out.
I then tried to allocate a drive letter and formatting. Then I see it as a new drive but when I try extend C drive it is still greyed out.