The “Edit Dash Style” dialog box, only has “New”, “Edit”, and “Delete”

However, there is a method for duplicating the settings in an already existing dash style in order to construct a new one without having to start from scratch: 

1) Go File > Document Settings > Dash Styles.

2) Select the existing dash style that you would like to duplicate.

3) Click the “New” button.

 

4) The dash style that is loaded by default in the one you had selected. Now simply make the changes that you would like, and click OK.

5) Now, you have a “New” dash style that is based on the old one.