Garage surface conduit feed an outdoor outlet
THHN must be entirely contained within conduit...
Your approach won't work because NEC 310.10 requires that you use insulated conductors as part of a Chapter 3 wiring method, instead of standing on their own.
but, you can use a stub length of conduit from the body to the box
However, outdoor-rated boxes generally have hubs built into them that accept conduits of an appropriate size.  As a result, you can fit a stub length of PVC conduit (often called a &quot;nipple&quot;) between the conduit body and the box, provided you're using a plastic weatherproof box.  (These are often called &quot;FS&quot; or &quot;FD&quot; boxes.)