As we all expected, Pinnacle Studio 9 Plus will not let you mix 16:9 and 4:3 videos without "converting" one kind to the other. No matter how much you love your program, it is not going to do what you want. AVS Video Conterter will not help in the least since you're not trying to change formats. There is no 16:9 video codec (read format.) Codecs generally don't care about aspect ratios. So far as I've been able to confirm, video files have a "wrapper" that tells the codec how to display the video data. Your program that reads the video format sounds like it doesn't read the "wrapper." Your NLE does read the "wrapper" but CANNOT mix aspect ratios on your timeline.
I have copied a couple of quotes from your manual that tell you exactly that. Your experience proves they are right. (Odd, the manufacturer being correct about what it will actually do.) If you want to mix the aspect ratios on a single timeline, YOU HAVE TO USE ANOTHER NLE. Now I'm to lazy to find out if you can upgrade to a Pinnacle product that does allow mixing aspect ratios. Well not so much lazy as I don't care. If you insist on onl;y using your current NLE, we can provide no advice as to how you can mix aspect ratios. However, we know of dozens of NLE's that take this stuff in stride.
Here are the quotes from your software manual. Believe what they tell you.
Studio does not allow you to mix footage of different
aspect ratios in the same movie. If you want to add
16:9 footage to a 4:3 movie, or 4:3 footage to a 16:9
movie, you must first use whichever Aspect Ratio
command is needed to bring the Album scenes into
conformity with the movie. The new clips will be
squeezed or stretched as needed to match the movie.
Because any given movie can be in only one of the
standard 4:3 format and the widescreen 16:9 format,
Studio does not let you mix frame formats in the Movie
Window. The first video clip you add to a movie
determines the movie’s frame format, and later clips
must conform to it. Use the Aspect ratio commands on
the Album menu to convert scenes from one format to
the other. (See “Video aspect ratios†on page 42 for
more information.)
When you are convinced you cannot do what the manual tells you that you can't do, you can get our help in selecting an easy to use NLE that does let you mix aspect ratios in a single timeline. Or you might be able to use the interface you are familiar with by upgrading to another Pinnacle NLE.
If you are having difficulty in finding the Pinnacle NLE that will do what you desire, I'm sure the forum members can provide that information. Good luck and happy editing.