Yes, this is an option. In this case, you would manage your corporate website within your primary CMS, while managing your portal site using Sitefinity. You would use The Portal Connector to build out forms, grids and pages Your Portal would act as a stand-alone site (ie. portal.yourcompany.com), but it would match the look and feel of your website, providing a seamless experience for your users. Alternatively, you could create the portal elements and embed each element into your site using iFrames and the seamless.js library or something similar.
Note that approximately 9 out of 10 of our customers who go this route ultimately decide to move to Sitefinity altogether because of how easy it is to use, how full-featured it is as an enterprise class CMS, and how affordable it is relative to some other CMS options.