To add hair to a blend shapes 3D model without Auto-rig, manually create hair geometry (strands or cards) and link it to the model via vertex groups or weight painting—this ensures hair follows blend shape deformations.
Design hair as separate meshes matching your model’s style. In software like Blender or Maya, use vertex groups to attach hair vertices to the model’s head: when blend shapes (e.g., smiles) change the model’s shape, the hair moves with it. Weight painting lets you control how tightly hair adheres to different areas, preventing clipping.
For Blender users, try the Hair Particle System to generate procedural hair, then convert it to a mesh. Use “Weight Paint” mode to paint the hair onto the model’s head vertex group—this is a simple way to sync hair with blend shapes without auto-rigging.