Hello! The PBR Material already has a lightmapTexture property: PBRMaterial | Babylon.js Documentation (babylonjs.com), so after loading the material you could simply assign the texture you baked to the lightmap.
This topic might also be useful: Baking Bake Lightmaps Lightmapping (The Texture Kitchen) - Tutorials and tips - Babylon.js (babylonjs.com)