You post title and your question is conflicting. Do you want to login to moodle from joomla backend? If so, you cannot. Joomdle is a student application not an admin one.
Is it possible to have Joomdle log you into Moodle when you login to Joomla. Yes this is possible. Please read through the Joomdle features so you have a good understanding what Joomdle can help you with. Thus you will know if it the right tool for you.
