The Custom legend can add color tiles, points, and patterns, but does not have a built-in line/symbol option.
A way to do this is to create some Plot commands that are hidden from view. You can use a group to hide them all.
Then add each command as an entry in the custom legend, where you Present the ‘Default’ legend (the line and symbol).
Then your graph would like like this.
Of course, in this case, you can get the same look with a the standard Legend command.
Curious as to why you want to use the Custom legend rather than the standard Legend?