You can use the NationBuilder theme editor to transfer over your HTML and CSS to match the look and feel of your NationBuilder website with your existing site. However, NationBuilder has many different types of "pages" where features and best practices of specific activities (such as petitions or donating) are built in. Recreating these pages is quick and easy and they will inherit the design you configured in the theme editor. You also have full access to the HTML and CSS of every page.
If you don't have the time or ability to do this, we can refer you to consultants experienced in customizing NationBuilder for you. Just drop us an email.