Ok...I think what you are looking for is block insert mode. So assuming you are in Vim, navigate to the area where you want to start your selection and, in command mode, hit Ctrl + v. Now you are creating a block selection...so just use the normal h,j,k,l movements (or arrows if you like) to highlight the desired region. If you really want to use the mouse, you can just right-click and drag in the editor to create the selection.
Now toggle the SlickEdit command line with Ctrl + a and invoke block-insert-mode, and start typing. Is that it?
- Ryan