When you do Pkg.add of a local path, it will pick up what’s committed to git and ignore local changes. When you do development of a package you should almost always either use Pkg.develop or activate the environment of the package itself.
What is in that Project.toml file? Does it have a name entry?
In my fork was no file Project.toml included.
What is the clean way to add to my fork a proper Project.toml?
This question from you was a trigger 
I have copied the Project.toml from the original package and replaced the uuid.
The new uuid was generated via the command:
import UUIDs; UUIDs.uuid4()
Now I was able to send the command:
Pkg.develop(path=raw"C:\folder\to\my\local\fork\EquivalentCircuits.jl")
Regarding this, I have a question:
I guess this command links / adds this fork (forked package)
to the current environment and this command is a replacement / alternative to the Pkg-command:
Pkg.add() Is this correct?