CHAMP1If they are POP, you can use a Global Inbox (aka Local Folders)
Or you can nominate any folder to act as inbox.
Tools|Account Settings|<select account>|Server Settings→Advanced
If there are any IMAP accounts involved, then the Unified Folders view (View|Folders|Unified) provides a reasonable approximation to a combined inbox.
CHAMP0Write a Message Filter
Copy to Local Folders|Inbox
But rather than copying and duplicating the amount of HDD space being used, you could set up a saved search, where you create a virtual folder made up of selected folders.
Open your Local Folders, ctrl+shift+f, do a search that will pick up all messages (“from” “doesn't contain” “IDontBelieveItOrSomeOtherStringThatWillNeverHappen” for instance), select Saved Search and add in the required other folders.