Assuming you know PHP, there are a number of tools to assist you, but here are the general steps you'd use to get your static content into a Joomla database. In short, each page you import will be an article in the Joomla table #__content. It's important that you use the Joomla models, as there are a great deal of database relationships that must be correct (i.e. Permissions, tags, searches, etc.) that the Model takes care of.
Crawl your site: Assuming you don't want to manually process each page, I've used PHPCrawl to crawl our sites to find the pages I'm interested in
Parse the content from your HTML With each page that you want imported, you'll need to parse the HTML of that page to locate the content you want on the Joomla site. There are a number of ways to do this, but this question/answer will give you a starting point if you don't have your own ideas.
Create Categories: Depending on how many categories you have, and whether you want to create them manually, or automatically based on your extracted information above, this code will create a new Joomla Category from PHP. There is more data that is needed for a category, but you get the idea:
$basePath = JPATH_ADMINISTRATOR.'/components/com_categories';
require_once $basePath.'/models/category.php';
$catmodel = new CategoriesModelCategory(array('table_path' => $basePath.'/tables'));
$catData = array();
$catData['id'] = 0;
$catData['parent_id'] = 'your_parent_category_id';
$catData['title'] = 'your_category_title';
... continue with remaining data from #__categories
if(!$catmodel->save($catData)){
echo 'Saved Category';
}
Create Articles: For each page that you parse, you then create a Joomla article to contain the HTML. That HTML can/should be split into the introtext or fulltext columns, but that's up to you. Creating an article is similiar to creating a category.
$basePath = JPATH_ADMINISTRATOR.'/components/com_content';
require_once $basePath.'/models/article.php';
$article_model = new ContentModelArticle(array('table_path' => $basePath.'/tables'));
$articleData = array();
$articleData ['id'] = 0;
$articleData ['catid'] = 'the_category_id_of_the_article';
$articleData ['title'] = 'your_category_title';
$articleData ['introtext'] = 'the_HTML_parsed_from_your_webpage';
... continue with remaining data from #__content
if(!$article_model ->save($articleData )){
echo 'Saved Article';
}