Entity Framework Core: Extend Entity without partial

I have to extend current database entity (Foo) to include new fields. Sounds easy but I struggle with solution. Problem is, I'm not able to create a partial class for Foo because it's part of a dll which I'm using. And there is no way how I could request modify Foo (or do it by my own) as I wish.

My project structure is similar to this:

// unmodifiable because part of dll
class Foo 
{
    int Id { get; set; }
    string Name { get; set; }
}

// unmodifiable because part of dll
interface IFooContext 
{
     DbSet<Foo> Foo { get; set; }
}

class FooContext: DbContext, IFooContext 
{
     DbSet<Foo> Foo { get; set; }
}

With my limited knowledge of .net, what I tried, was create class Bar what extends Foo and expected it would be enough to implement IFooContext with extended class Bar.

Below you can see, but it will end up with error, because C# doesn't allow use extended class interchangeable for parent.

class Bar: Foo 
{
     string Description { get; set; } //new field
}

class FooLoader: DbContext, IFooContext 
{
     DbSet<Bar> Foo; //error because DbSet<Foo> required
}

What I could do, is create property DbSet<Bar> Bar { get; set; } inside FooLoader, but then I will end up with two almost identical database tables: Foo and Bar.

So I made modification and created class Bar which will contain FooId. It results into two tables, where Bar table has a reference to Foo.

class Bar 
{
     int Id { get; set; }

     Foo Foo { get; set; }
     int FooId { get; set; }

     string Description { get; set; }
}

Both cases, will create a new table what I'm not sure, if is what I looking for. But is there any other way how to do it and have new fields included in base class/table?