Can I get the last word of Slow Time without joining the Stormcloaks?

Solution 1:

You can not enter Korvanjund without siding with some faction in the civil war. That means either joining the Stormcloaks or the Imperials is your only way to get that Word of Power.

Solution 2:

You can get the 3rd slow time word without joining any civil war factions by exploiting the plate/bucket glitch that people use to teleport through walls. With a few wooden plates you can glitch through the rubble at the start of the dungeon, swing by the word wall, and glitch back through the rubble. (I'm not sure this won't permanently break the civil war quest, so I wouldn't do it unless you want the imperials and stormcloaks to remain in a stalemate.)