How do I add an electric fireplace insert in a entertainment center?

Well I am sure you can, there will be some retro fitting required to make it fire safe. Shop your big box home improvement centers, hardware stores and fireplace shops. Pay close attention to energy usage as some of the cheaply made models suck up electricity like crazy. Check installation instructions carefully too. Will look very pretty when you have it all set up.

http://www.electricfireplacesdirect.com/plug-in-electric-fireplaces.html

I found this for you:

http://www.electricfireplacesdirect.com/blog/How-To-Install-An-Electric-Fireplace-Insert

Hi TP, you can buy fireplce inserts at several places, I think Walmart even sells them, here's one style from wayfair

https://www.wayfair.ca/home-improvement/pdp/charlton-home-gilcrease-electric-fireplace-insert-c000276517.html?piid=515485597

And Walmart

https://www.walmart.com/browse/home-improvement/fireplace-inserts/1072864_133032_5411540_3097034

Very doable !!! After my last electric fireplace finally passed away (a throw out on the side of the road), I searched and searched for just the right one. In style, size AND the heating element on the top since my granddaughter is crawling. Make sure to measure the opening and have enough air circulation. Always check out the comments on the sites you check out, very helpful. Oh, and mine came with a remote !!! Good luck!!!

I found this guide useful. https://magikflame.com/learning-center/home-improvement/can-you-put-an-electric-fireplace-in-a-bathroom/

Measure the space that you plan to insert the fireplace. Find one as close to the size of the opening as possible. Be sure it is not too big. We had to buy one a bit smaller than the opening as the opening in ours was smaller in height than yours looks.

We got this just before Christmas and we love it. Room was a bit crowded due to the Christmas tree.

We just placed it in the opening after we removed the doors and plugged it in.

We are not planning on using the heating option, we just wanted the esthetic of the fire. Hope the picture works....