I got it!
After looking the source, I found this function : function
SetCustomWritePanel($postId); [1] that did the trick.
When saving a post, this function is invoked (this is the way
assigning a writepanel to a created-before-installing-magic_fields
post works).
So what I did was change my WXR files (Wordpress XML exported files)
and added to each item by hand (well, using a simple script) the
following tags:
<wp:postmeta>
<wp:meta_key><?php echo CUSTOM_PANEL_KEY ;?></wp:meta_key>
<wp:meta_value><?php echo CUSTOM_ITEM_TYPE ;?></
wp:meta_value>
</wp:postmeta>
where CUSTOM_PANEL_KEY is "_mf_write_panel_id" and CUSTOM_ITEM_TYPE
the id of the custom panel I want to add to this type.
So if anyone needs to massive add previous items to Magic Fields, you
can do it that way :D
[1]
http://github.com/hunk/Magic-Fields/blob/master/RCCWP_Post.php#L24
Thanks for your great plugin!