Here's a bit of code for when I imported my old blog content into v8, should help, mainly the last part I guess - make sure to get a stream from the file and then you can indeed use the MediaService to save it:

    public GuidUdi CreateMediaImageFromMediaId(int mediaId, IEnumerable<MediaItem> mediaItems, string nodeName)
    {
        var mediaFile = mediaItems.FirstOrDefault(x => x.Id == mediaId);
        if (mediaFile == null)
            return null;

        var url = $"https://cultiv.nl/{mediaFile.UmbracoFile}";

        return CreateMediaImageFromUrl(url, nodeName);
    }

    private GuidUdi CreateMediaImageFromUrl(string url, string nodeName)
    {
        var downloadPath = IOHelper.MapPath("~/App_Data/TEMP/DownloadedFiles");
        if (Directory.Exists(downloadPath) == false)
            Directory.CreateDirectory(downloadPath);

        var fileName = url.Substring(url.LastIndexOf('/') + 1);
        if (fileName.Contains("?"))
            fileName = fileName.Substring(0, fileName.IndexOf("?"));

        var filePath = $"{downloadPath}/{fileName}";
        if (File.Exists(filePath) == false)
            using (var client = new WebClient())
                try
                {
                    client.DownloadFile(url, filePath);
                }
                catch (Exception e)
                {

                }

        if (File.Exists(filePath) == false)
            return null;

        var blogPostsRoot = _helper.MediaAtRoot().FirstOrDefault(x => x.Name == "Blog Posts");
        Guid blogPostsRootFolderId;
        if (blogPostsRoot == null)
        {
            var blogPostsRootFolder = _mediaService.CreateMedia("Blog Posts", -1, "Folder");
            _mediaService.Save(blogPostsRootFolder, userId: -1);
            blogPostsRootFolderId = blogPostsRootFolder.Key;
        }
        else
        {
            blogPostsRootFolderId = blogPostsRoot.Key;
        }

        var thisBlogPostRoot = _helper.Media(blogPostsRootFolderId).Children.FirstOrDefault(x => x.Name == nodeName);
        Guid thisBlogPostsRootFolderId;
        if (thisBlogPostRoot == null)
        {
            var thisBlogPostsRootFolder = _mediaService.CreateMedia(nodeName, blogPostsRootFolderId, "Folder");
            _mediaService.Save(thisBlogPostsRootFolder, userId: -1);
            thisBlogPostsRootFolderId = thisBlogPostsRootFolder.Key;
        }
        else
        {
            thisBlogPostsRootFolderId = thisBlogPostRoot.Key;
        }

        using (var imageStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
        {
            // Stream the file into a new umbraco media item
            var imageMedia = _mediaService.CreateMedia(fileName, thisBlogPostsRootFolderId, "Image");
            imageMedia.SetValue(_contentTypeBaseServiceProvider, "umbracoFile", fileName, imageStream);
            _mediaService.Save(imageMedia, -1);
            return new GuidUdi("media", imageMedia.Key);
        }
    }

Thanks Sebastiaan

Having a difficulty crossing this 'lines'.

_mediaService and _contentTypeBaseServiceProvider declaration from y ContentService.IUserComposer extension

is working on a reply...