When signing up as an organisation, you'll have the option to claim your Do-it.org organisation and create a new Do IT account.
In the screen above, click on the 'Do-it.org' text.
Here, sign in with your current Do-it.org credentials. Following this, you'll end up at a screen where you set the details for your new Do IT account (you'll be signing in from now with the email address of your old Do-it.org account).
On the following screen you'll see organisations you're able to claim. Select the organisation and click 'Next'. Please note that currently only live opportunities you had on Do-it.org will sync over.