How can I use the MageMail email template from within Magento transactional emails?
If you want to use a similar HTML email template as the MageMail template for your Magento transactional emails, for one thing you can use MageMail's transactional emails.
But beyond that, if you'd like to use the MageMail template for other transactional emails that aren't supported currently, you can simply copy the HTML/CSS for a MageMail email and adapt that to your Magento email templates.
The best way to do that is to simply look at one of the emails sent in the Recipients tab under a given email trigger, click to view it, and then grab the contents of the HTML and modify it.
I could also show you the source files that I use within the MageMail app but they wouldn't be as helpful to you because the CSS is automatically inlined - so you would have to have an inlining capability on your end to be able to use them.