You can't. But you can reduce the line spacing for the whole paragraph by a small multiple (e.g. instead of single line spacing, set the multiple to 0.95). Visually, too, that would also be less noticeable than having just two lines with a different spacing. An alternative would be to insert an ADVANCE field at the start of the first line you want to modify and another at the end of the last-modified line to undo the first field's effects. See:
Field codes: Advance field - Office Support