I have nikon D3400 and an extra sigma lens 70-300mm DL Macro Super. They are not compatible because the lens does not have a built in af motor and d3400 also does not have af motor in it. Is there any extension or adapter that would make these 2 compatible? THANKS
Your left hand is the only adapter available to focus that lens on that camera. Before the mid-1980s, everyone had to focus that way.
Here are some questions regarding doing manual focus with the cameras we had back then as well as with more modern DSLRs:
How did photography work before auto-focus was invented?
Can viewfinder type make an appreciable difference in manual focus?
Why don't DSLRs come equipped with "classic" focusing screens (split prism, etc)?
Disadvantages of split-prism focusing screens?
Do focusing screens exist for modern DSLRs?
What is a focusing screen?
What are the differences between different types of focusing screens?
Supermoon: In focus and Out of focus
This answer to What are some alternatives if you don't have a flip screen in your camera?
How do you manually focus accurately without a focusing screen?
Catadioptric lens: equivalent focal length and focus at infinity
Why do some lenses focus past infinity?
Why are photos taken with my old lenses all out of focus on my new Nikon D5300 body?
Does the Sigma 70-300 AF-D work on the Nikon D3200?
Will a AF VR_Nikkor 80-400mm 1:4.5-5.6D lens work on a Nikon D3400?