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

Greets