[kh99]

Well, I'm not a SQL expert (just learning actually), but I ran a little test and this seems to work (use at your own risk!):

UPDATE vb_post AS post 
  LEFT JOIN vb_addings as addings 
  ON (post.postid=addings.postid AND post.threadid=addings.threadid) 
  SET post.pagetext=CONCAT(addings.addtext,  post.pagetext)   
  WHERE addings.addtext IS NOT NULL

Notes:
1. This seems very dangerous! Are you sure you want to try this? Back up your database first.
2. The 'vb_' in red should be changed to be your own prefix.
3. Did you add a prefix to your 'addings' table? Change or remove the 'vb_' in blue accordingly
4. I test both postid and thread id, although I believe postid is unique
5. If you want to put something between the addtext and pagetext, you can add it to the CONCAT function (in green), such as CONCAT(addings.addtext, ' ', post.pagetext) to add a space.
6. Are you sure? Maybe run a test with an addings table that has only one or two rows.