Tell that to Bush, he's been Adding So-called New Laws by EO & adding Write ins to Laws/Bills he signs.
A couple of things about executive orders and signing statements.
Executive orders are neither illegal, unconstitutional, or immoral. EVERY president since GW has used them. They are how the president implements the laws congress passes.
Signing statements have also been around a long time. Most of the time they indicate how the president views vague or unconstitutional clauses of the law. How else are executive branch employees to decide what they mean? If they are unclear, its up to the president to clarify them. One of the more infamous signing statements had to do with a border security bill. Some congressman had a stipulation put into law that a specific border station was to be manned 24/7. GWB signed the thing into law, noting with a signing statement that (quite properly in my mind) such a directive from congress would be exercising executive power rather than legislative power, so essentially he planned to man the border station as he saw fit.