Those prefab coops are not a great base to add on to. They're small, flimsy, made of cheap materials, and don't have enough ventilation. It looks like you're thinking ahead on this, so that's good! Here are some thoughts/questions to get some more info so people can give better advice.
Can you get some pictures of your set up as it is now? How close to another building/fence/trees, etc.
Where do you live? Your climate matters a great deal for building a coop, even if it's modifying an existing one.
What will you use for the walls? What kind of predators do you have to deal with?
Possibly doing hardware cloth floor with sand over it as a floor.
Chickens like to scratch. The hardware cloth is not going to be a good floor for them, even if you cover it with sand. Take the hardware cloth and use it to make an apron around the run instead to keep out predators. Depending on your climate, sand may be a great floor, or terrible.
A starting point for space is:
4 square feet per chicken in the coop
10 square feet per chicken in the run
1 linear foot of roost per chicken
1 square foot of ventilation that is open 24/7/365 per chicken. (Open means covered with hardware cloth to prevent predators from getting in.)
There are lot of articles on building/modifying coops. Read through some of those and you might get a better idea of what to do with what you have.
And, welcome to BYC.