Yes you can, the Last Wish and Garden of Salvation secret chests can drop the raid armor and weapons without ever completing a single encounter in the raid.

For the most part, a secret chest is only gonna drop armor that you've unlocked before... However, as a previous reply mentioned, Garden & Last Wish for some reason will drop armor that you have not unlocked before. Not sure if its meant to be that way, or if its some bug... When it happened to me, I was running my Warlock thru Last Wish. I'm a Hunter main & have all the stuff unlocked on Hunter, so idk if it counted as "unlocked" for the Warlock too... or whether a completely new account with zero raid clears can still get armor from Garden & Last Wish secret chests.