Hi. My name is*****'m a camera repair technician with almost 30 years in the business and I'll be glad to help you with your question today.
Just to clarify, you have screw-mount lenses you want to use on a Nikon FG, is that right?
Just a note to see if I can be of any further assistance?
I'm sorry but what you need is a macro lens that fits the Nikon FG. Nikon makes several of these in different focal lengths that sell very inexpensively on the internet or from your local camera store. One place I know of with a good supply of macro lenses that will fit your camera is :