public class MyClass
{
  public BigInteger Value { get; set; }

  public Dictionary<BigInteger, string> Dictionary { get; set; } = new Dictionary<BigInteger, string>();
}

public class BigIntegerConverter : TypeConverter
{
  public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
  {
    if(value is string)
    {
      return BigInteger.Parse((string)value);
    }
    return base.ConvertFrom(context, culture, value);
  }
  public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
  {
    if(sourceType == typeof(string))
    {
      return true;
    }
      return base.CanConvertFrom(context, sourceType);
  }
}