how to keep the texture of a obj file after modifying it?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hey every one

I have a pointcloud in .obj format and I want to modify it like add some lines and points to it but want to keep the texture of the original file the same it was before the modification. how can I do that using open3d?