Hello,
sitemap generator should start crawling your pages from the start next time, otherwise it will be unable to detect if there are any new pages on your site (i.e. it crawls your site following all links found, starting from homepage).
In case if your fresh content is limited to a specific section of your site that can be detected via URL, I'd recommend to keep existing full sitemap unchanged and configure sitemap generator to create an additional sitemap for that area *only*, using "Exclude URLs" option.