Yes, you can use Margin effect to dilate a segment by a margin. You probably want to make a copy of the GTV before you grow it, and set Masking/Modify other segments to “Allow overlap” to allow the GTV and PTV segments to overlap.
If you want to restrict margin growing to be inside the lungs, you can segment the lungs using Lung CT segmenter module (provided by LungCTAnalyzer extension), import the lung segment into your segmentation, and choose it as Editable area in Masking section when you use the Margin effect.