One way to do this (and I don't know of any pen and pencil kits that are specifically designed to your requirements) ....
There are some kits that allow you to replace the ink refill with a pencil refill cartridge.
It might be the case (but I don't know) that cartridges in both the 0.5mm and 0.7mm sizes are available.
One kit that offers (or, at least, offered in the past) some of this is the El Grande. I remember making both pen and pencil versions, but I don't remember what size the pencil lead was. As I recall, the operation mechanism was of the twist type.