Импортируйте элемент со стилями. Режим: KeepDifferentStyles

using SautinSoft.Document;
using SautinSoft.Document.Tables;
using System.Linq;

namespace Sample
{
    class Sample
    {

        static void Main(string[] args)
        {
            // Get your free 100-day key here:   
            // https://sautinsoft.com/start-for-free/

            ImportKeepDifferentStyles();
        }

        /// <summary>
        /// Import an Element with Styles from another document. Mode: KeepDifferentStyles.
        /// </summary>
        /// <remarks>
        /// Details: https://www.sautinsoft.com/products/document/help/net/developer-guide/import-element-keep-different-styles.php
        /// </remarks>
        private static void ImportKeepDifferentStyles()
        {
            // Mode: KeepDifferentStyles 
            // The most useful mode to preserve formatting during importing.

            // 'KeepDifferentStyles' means to only copy styles that are different by formatting.
            // If the destination document already contains a style with the same name, 
            // therefore an unique style name will be generated.

            // For example, a destination document contains a style "MyGreen" (FontSize = 20, Green, Underline).
            // And a source document also contains a style with name "Green" (FontSize = 20, Green, Underline).
            // Because of the formatting of styles are equal, the "Green" style will not be imported.
            // All imported elements linked to style "Green" will be remapped to style "MyGreen"

            DocumentCore source = DocumentCore.Load(@"..\..\..\SourceStyles.docx");
            DocumentCore dest = new DocumentCore();

            // Create a new style "MyGreen" (FontSize = 20, Green, Underline).
            CharacterStyle chStyle = new CharacterStyle("MyGreen");
            chStyle.CharacterFormat.FontColor = Color.Green;
            chStyle.CharacterFormat.Size = 20;
            chStyle.CharacterFormat.FontName = "Calibri";
            chStyle.CharacterFormat.UnderlineStyle = UnderlineType.Single;
            dest.Styles.Add(chStyle);
            dest.Content.End.Insert(new Run(dest, "This text has the style MyGreen.", new CharacterFormat() { Style = chStyle }).Content);

            // Create an ImportSession with mode 'KeepDifferentStyles'.
            ImportSession session = new ImportSession(source, dest, StyleImportingMode.KeepDifferentStyles);

            // Let's import a paragraph.
            // The imported paragraph contains a text with style "Green" (FontSize = 20, Green, Underline). 
            // The style "Green" will not be imported, because we already have "MyGreen" with the same formatting.
            // All links to "Green" will be remapped to "MyGreen".
            Paragraph importedPar = dest.Import<Paragraph>((Paragraph)source.Sections[0].Blocks[0], true, session);
            dest.Content.End.Insert(importedPar.Content);                  

            // Save the destination document into DOCX format.
            string docPath = "KeepDifferentStyles.docx";
            dest.Save(docPath);

            // Open the result for demonstration purposes.
            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo(docPath) { UseShellExecute = true });
        }    
    }
}

Download