Extremely late reply but for anyone wanting to do this in the future here’s a solution. The .mesh files Roblox uses (such as head.mesh) are not normal .mesh files, they only work on Roblox’s engine.
If you want to convert it you have to upload the mesh to Roblox like normal and copy the assetid. Then enter the link in your browser as shown below but add the copied assetid at the end:
https://assetdelivery.roblox.com/v1/asset/?ID=(enter your assetid here)
A download should start and rename the file to whatever you want. But note it must have the same name as any other mesh in the Roblox content folder you want to replace, such as a body part in the avatar folder.