Workaround: Handle the CommandExecuted event and change the font family after executing the paste command:

private void RadRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
{
    if (e.Command is PasteCommand)
    {
        foreach (var p in this.radRichTextEditor1.Document.EnumerateChildrenOfType<Paragraph>())
        {
            if (p.IsInList)
            {
                var list = this.radRichTextEditor1.Document.ListManager.GetDocumentListById(p.ListId);
                list.ActualStyle.Levels[0].FontFamily = new Telerik.WinControls.RichTextEditor.UI.FontFamily("Symbol");
            }
        }
    }
}