For every link contained in a post, q2a adds the rel=nofollow attribute *by default*. So you do not have to worry.
On the contrary, you'd like to convert your internal links (those pointing from a page to another page within your website) to have a "dofollow" relationship, so that they help in passing the link juice equally to other pages within your website.
To accomplish this conversion, you could use for instance the Q2A-Ultimate-SEO plugin or the previous Q2A-SEO-Links plugin .