i mean ultimstely you can downlod a 2d hair texture, import to roblox, put it as a texture on the mesh, then make the mesh transparent. i do that alot when modeling trees with blender/roblox.
Roblox doesn’t support multiple materials being used for one mesh, so if that’s what you’re doing, you should separate the hair and body to two meshes.
Also, have you tried manually importing the hair texture through the Asset Manager?
In edit mode, select all the faces for the hair planes (so select all of the hair). Then press P > Selection.
This will separate the selected faces based on the selected faces into two meshes.
If the hair uses a different material than the body then while in edit mode click on the materials panel, select the hair material then click “select”. That will select all faces related to that material.