Could I use a felting needle to join the edges of felt that was made by knitting, then shrinking?

I think you could. I do dome needle felting and I felt items onto felted items and that works.

What type of item is it? Will there be stress to this area you are felting together?

I don't think there will be much stress on the felt. I was thinking of using this for the white edge on my slippers, instead of making one wide band to felt and hope it turns out the right size.
By the way, I have never used a felting needle before.

Felting needle is very easy to work with but extremely sharp so be careful!!!!!!!!!!

Also you only need one and felting needle will not felt the item exactly. It will cause the fibers to mix. So felt the material first and them attach it to the other item using a felting needle.

Thanks a lot! I thought that was how I should do it.