I have observed your requirements and like to share that text frames display text data of the shape. Internally, a text frame has got collections of paragraphs. Each paragraph has got collection of portions inside it. There are certain textual properties like text italicizing, boldness, underlining, height, font family and color that are available on portion level. On paragraph level there are different properties like bullets, spacing etc. These are set w.r.t their implementation in DOM (Document Object Model) of presentation that PowerPoint understands. We cannot have a paragraph level property used for portion level.
Now, as a workaround for your scenario you may add a paragraph with only one portion inside it. In this way, when you will apply the spacing property on paragraph level, it will be applied to only single portion inside text.
Please also remember that portions inside a paragraph are determined based on any change in any of above mentioned portion related properties. Otherwise, if there is a single formatting for all text inside portion, then a paragraph will have only portion inside it.