I have a object and I want to add another transform to it, localy… How?
I guess if you want to apply another transform to your own, you could do something like this:
Transform a = transform;
Transform b = someOtherTransform;
a.position = a.position + b.position;
a.rotation = a.rotation * b.rotation;
a.localScale.Scale(b.localScale);
But I think that is an unusual approach, because a transform is always attached to another GameObject. You would rather store the rotation, scale, etc. in another way.
Use a script.
Something like
public transform yourObject;
public transform yourObject2;