Hi Pavius,
you can't edit a package in the library. Attempting to add or remove pads will result in the error message "Package in use".
This is very logical. Editing a package will have effect on all devices sharing that package - resulting in most likely pin/pad errors in the schematic and PCB layout.
If your package is similar to the one found copy it using another name. Load it into the editor and you're free to add or delete pads, resize, reshape and renumber them as well as changing the package dimension.
The same applies to the symbol.
When both - package and symbol are completed create a device using the symbol first and assign the new package.
Here is an example for a 4-digit LED display, one to create a PCB for manufacturing and the other one for representation purposes. (3641F was created by copying 3641)
Boncuk