I guess you mean additional pages. You can always add portfolio items of various categories (illustrations, photography etc.) and then display them all in the portfolio pages currently. If you need to create additional portfolio pages dedicated to certain categories only, add a regular of template “One Column No sidebars” page and use the use the shortcode
[show_post_snippets post_type="portfolio" number_of_columns=4 post_count=8 image_size="small" taxonamy= 'portfolio_category' terms='illustrations']
Replace terms with your desired category like photography etc. You will just need to have the appropriate category set when you add a new portfolio. Hope this helps.