Hope you can help!

I have produced a complete website in dreamweaver and now i want my client to use Coffecup so they can update the site content themselves.

I have an external linked CSS file that controll the look and feel of the site but when i imported the site into coffecup i cannot see a way where they (client) can just click on existing css attributes to their new content additions.

Coffee cup just shows the complete tags list that will then insert code into the pages code - defeating the purpose of the linked css file - is there a way i can get this to work with custom tags from the external css file and that all the attributes of the page show up in the list?

Love the simplicity of Coffecup and their family of products.

CoffeeCup will allow you to edit the .css directly within the HTML editor, but it does not do the same thing as dreamweaver. The Visual site designer builds a style sheet on the pages themselves, but it won't edit an existing style sheet or existing website.
Dreamweaver wrote the style sheet to go with the web page (am I right?). CoffeeCup is an editor to edit code, and it can't do things exactly like dreamweaver. Your client would have to learn some .css and edit that file to make changes to the layout or overall look and design of the fonts and such as they are already in a .css sheet.
In short, CoffeeCup HTML editor does not write a style sheet for you, but you can use it to edit any style sheet or build one yourself.

Would CoffeeCup's css editor be useful in this case?

The css editor is a nice tool, to build and edit .css files, but you still need an understanding of .css code and where/what changes to make to the .css file. I don't think it can help build a .css file out of a web page and attach that .css file automatically like dreamweaver does.