Not sure what your trying to say here. If someone places a comment you want us to actually search that comment for a link and if there is one add a no follow statement to the actual link inside the person's comment?
Feel free to place suggestions in the suggestions forum if you want to make a suggestion on a feature that does not exist in the actual program.
We have nofollow relation statements on any relative links in our actual program what your asking is some type of filter on user posted content.
For example I know how to alter our coding say for our convert_markups function to add a nofollow relation to links when we convert links thats easy. However if your using vbulletin we use the vbulletin bbcode markup functions and if they do not insert nofollow tags then there is not much I can do about there code.