Hello.

We create an extension with the J-Cook that uses JDOM she could not be published in JED,

JED Entries License Checklist

"List C3 - GPL compatible licenses

4-Entry uses Creative Commons, BSD, MIT or any other free software licenses:
Publishable? No. Only licenses que comply with all terms of the GPL will be considered. "


Question: the license in the header JDOM.PHP file can be modified MIT license to GPL license for which extension can be accepted and published in the Joomla! Extensions Directory - JED?

Thank you

You are right.
With the pure version, you will be able because JDom will not be anymore necessary, and the Formvalidator will not be anymore included (MIT license).