I ended up creating an extension method which access the base CoreBuilder to invoke AddFileSystemOperationDocumentStorage

public static class FusionGatewayBuilderExtensions
{
    public static FusionGatewayBuilder AddFileSystemOperationDocumentStorage(
        this FusionGatewayBuilder builder, string path)
    {
        ArgumentNullException.ThrowIfNull(builder);

        builder.CoreBuilder.AddFileSystemOperationDocumentStorage(path);
        return builder;
    }
}