Are you asking how to do it or whether it’s legal and ethical?
If you’re asking how to do it, given that you designed the site, I’m a little puzzled. Are you, perhaps, saying that you designed mockups of the site, but someone else coded and built it?
Copying website files and the associated databases are common procedures. Still, doing so involves technical expertise and direct access to the server through FTP or a site and database management tools like cPanel and phpMyAdmin. It also requires setting up a new site and database, then moving all the files and database information to the new location.
Given the nature of your question, I’m assuming you don’t have the kind of access to your employer’s web server that you’ll need, even if you possessed the technical know-how.
As for legal and ethical considerations, whether you designed it or not, the site belongs to your employer. You shouldn’t just copy, change and redeploy it somewhere else. First, that’s quite possibly grounds for losing your job, and second, it’s arguably illegal given that it’s their site, not yours.
If you’re grabbing a few pages for your portfolio, you still might need permission from your employer if the portfolio is publicly displayed. Assuming you get permission, demonstrating the site’s interactive elements would require a full-blown WordPress installation with your employer’s theme, which brings us back to whether you have the technical know-how to do it.
My suggestion is to use a few modified screen captures in your portfolio. Even if you can do everything I’ve just mentioned, I don’t think recreating a functioning website for a portfolio example is warranted or practical.
For what it’s worth, you’ve run into a common problem web designers complain about — watching as their initial designs slowly morph and degrade into a chaotic cacophony of bad design once the site is turned over to the client or employer to manage.