Legality of copying engine source to plugins

Sometimes, the editor functionality I need is not exported from the module. Since I can’t access it from the plugin, I copy that part over to my plugin code and rename the class.

Is this allowed by Epic?

P.S.: I provide the full source of the plugin to customers

Here is an example:
I want to add this FTransform customization

to my object’s transform, which looks like this by default:

This customization is provided by FComponentTransformDetails and is not exported, so I cannot access it from the plugin due to linking errors. I would like to copy it over to the plugin code