I figured it out on my own, yes you can extend from other extensions, you have to make sure that classes in other extensions are exported with PHPAPI and that the other extension is always loaded before yours.
extern PHPAPI zend_class_entry *class_entry_var;
**:EDIT if using below code then using PHPAPI is not necessary
and then you would refer to it in your class with
zend_register_internal_class_ex(&ce, NULL , "ClassName" TSRMLS_CC);