See valid_elements and extended_valid_elements.
EDIT:
You can use the private setting in tinymce initialization:
closed : /^(br|hr|input|meta|img|link|param|area)$/, // default
to
closed: /^(br|hr|input|meta|img|link|param|area|source)$/,
If this does not work, I think you may need to change one line of code from the main tinymce code located in the .js editor on line 2666 (the latest version of miny mce) and expand the setting there with the original tag.
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