Make Posted Sales Invoice Page editable true

My first question is why do you need to do this? I suspect you don't need to edit the whole page, but just selected values.

This page uses protected tables. Thus any permissions need to be assigned to the page. Unfortunately, the "permission" property is not available to Page Extensions (last time I checked). Therefore the direct answer is No. You would need to build a complete custom page. Which would bring its own problems.

If you only need to edit certain values, you could perhaps handle this thru some popup pages that allowed data entry. Then use code to update the real record when the page is closed.