Hi,
In share every form filed is rendered by a Controller.
You can find the OOB controllers in tomcat\webapps\share\WEB-INF\classes\alfresco\site-webscripts\org\alfresco\components\form\controls.
Now you need to implement a Custom Controller, that wolud have a button, onclick it would open create content Dialouge, onSuccess you wil get the created nodeRef, you assign this node ref to association field.
You can take a look into the association.ftl, how it is working.