Yes, you can transplant Plumeria seedlings outdoors once they have developed sturdy roots and true leaves. Before transplanting, harden off the seedlings by gradually exposing them to outdoor conditions over several days. Choose a sunny and sheltered location with well-draining soil for transplanting. Make sure the seedlings are adequately acclimated to prevent transplant shock. Plant the seedlings at the same depth as they were in the seed tray and water them thoroughly after transplanting. Protect the seedlings from strong winds and direct sunlight for the first few weeks to allow them to establish. Once the seedlings have acclimated and established, they can grow into mature Plumeria plants.