One way to do this would be to apply to a test page the built in Dreamweaver Insert Record and the Check New User Name using the same connection/table/column that you want to check in the "real" page. Then copy and paste the code for the Check New Username over to your real page.
This method will likely show a red exclamation mark next to the Check New Username in the Server Behaviors panel due to there not being a built in Dreamweaver Insert Record on the page, but it should work for you as long as you make sure to copy over all of the Check New Username code.