I would try to get it on this way.

Firstly ensure, that URLs are "speaking", so e.g. they look like `domain.com/category1/item1`, `domain.com/category2/item2`.

Then export all existing URLs into different files, which are sitemaps, by sorting URLs alphabetically, so each sitemap file gets URLs from single category (*category1.xml*, *category2.xml*)

It all can be done by simplest PHP-script, which reads all internal URLs and writes them into once created files. You can even create these sitemap-files manually, for each category.

Look here: http://php.net/manual/en/domdocument.save.php.