Hi all!

If those categories were added via Data Lists then it's possible to do the following MySQL query (backup the table `sys_form_pre_values` first):

INSERT INTO `sys_form_pre_values` (`Key`, `Value`, `Order`, `LKey`, `LKey2`)

SELECT 'bx_posts_cats', `Value`, `Order`, `LKey`, `LKey2` FROM `sys_form_pre_values` WHERE `Key`='bx_forum_cats';