You can, but it is a bit of a work around. (edited)
First, don't enter anything into the subject line. Just start your comment in the body. When you post the first part will appear as the title. Then go back and edit the post, selecting the first part using the html paragraph tag.
It's a pain, I know, but there is nothing I can do about it.
The subject line of a new thread starting post can be modified.