I want to archive all my current messages in Trash in Gmail, but can't find the archive button anywhere.
How do I archive trashed messages?
Only way I can think of is mark all with a label and put them back in your inbox then filter by the label you created select all and archive
Filter by typing
label: trashlabelname in the serach box, or the label should be on the left hand menu
Simply move them to Inbox first, to take 'em out of the Trash. Then proceed as usual.
If by any chance your inbox isn't empty (as it should), you should probably label them before sending to inbox, so you can easily select them then.
Every organization method in gmail, including folders, is actually just a label hidden or tweaked. Try creating a label "inbox", "trash" or "spam" and you'll see they're already used "by the system".
So, archiving is in reality just removing the inbox label.
But the system labels each work in its peculiar way and have their own reserved buttons to "add" or "remove" them. In case of trash, that (currently) is "Move to Inbox" which will remove the Trash label and add the Inbox.