Honestly WordPress should have this feature by default, you click on the link button in the top of the post to turn some images or text into a link and you get a popup that shows you 1 check box (open in new window).
But you should be making all links nofollow if they are linking outbound and especially for all of your affiliate links, and you may not feel like clicking on the text tab and putting rel=”nofollow” into the HTML code directly. You can use a simple plugin called Ultimate NoFollow for WordPress that simply adds a 2nd checkbox to your Link button window so that you can check mark nofollow and automatically add the rel=nofollow tag to your links.
Simply put it is a simple plugin that has one function and that is to make it a little easier to add nofollow to links if you use the WordPress post editor and the link button to create hyperlinks for images and text. This little plugin helps if you allow guest authors as well who write articles in your WordPress blog and you can remind them to nofollow all external links to better comply with Google ToS.
Ultimate NoFollow is a free WordPress plugin found in the WordPress archives, to install it you only need to browse the WordPress plugin directory from in your WordPress site by clicking on “Add New” then search for Ultimate NoFollow.
Simply click on Install now, then activate when installed.
Now the NoFollow Checkbox will show up when you use the WordPress post editor and click on the link button to link any images or text.