Get existing instance of List

I've inherited a bloated project that has been maintained by about six developers before I arrived here. This project takes 15 excel files as input, stores the values of those files in a huuuuuge class that serves as in-memory DB, runs a ton of calculations and returns about ten new files with the results of those calculations.

To try and make this testable, I tried adding a generic extension method to allow me to instantiate a class (this is no issue) and add those to an existing List.

I know how I can find the type of List I need using reflection, but what I don't know is how to return the already instantiated List of the in-memory database. Does anyone know if this is possible? I've added a quick example of this in-memory db below:

public class Database
{
    public class Parameter
    {
        public string Code { get; set; }
        public string Label { get; set; }
        public List<ParameterValue> paramValues;
    }
    public class ParameterValue
    {
        public string Value { get; set;}
        public Parameter parameter { get; set;}
    }
    public List<Parameter> parameters;
}

Obviously this class is hugely simplified (the Database defines several different Parameter classes and has most of those classes as List properties), but my question really is this:
Is it possible to get an already instantiated List<T> from a Database instance using reflection, if given T?

My apologies in advance if this has already been answered - I searched but didn't find an answer.

Robrecht