Is it possible to add Zermatt to 5 day Switzerland Trip???

From the BO you can get to Zermatt if you have a car...pretty long day though!

Drive to Kandersteg, take the car on the train through the mountain tunnel to Lotschberg (22CHF one way - really fun actually), drive the hour or so to the parking garage in Tasch, and then take the train up into Zermatt.
The problem is the weather. If you happen to be there on a cloudy/rainy day then you've wasted a day - it's a matter of luck unless you can watch the weather and go on a day when it is going to be clear. Even then you want to get to Zermatt as early in the day as possible as the clouds often build up in the afternoon, obstructing the views.
You can certainly make it there and back and the drive is lovely but it's a bit of an ordeal!

It is only 2 hours 40 minutes by train from Lauterbrunnen to Zermatt. So you "could" do a daytrip, but that is risky for seeing the Matterhorn as it is often clouded over. Why not do 3 nights Berner Oberland,,train to Zermatt for an overnight, then train to Lucerne for one night before flying home? The train from Zermatt to Lucerne takes 3 hours 20 minutes so you would have a whole afternoon and evening there.

I've been to Lauterbrunnen, Murren, Wengen, etc many many times. I had always been "itching" to see Zermatt and the Matterhorn so on one of our trips we did just that. I can't tell you what a let-down I experienced with Zermatt. After having been to the beautiful Berner Oberland so many times Zermatt just didn't hold up to the beauty I was expecting. It's touristy beyond belief and super expensive shops all over the place. It just didn't live up to the hype. Believe me, your time in the BO and Lucerne is exactly right. Don't waste your time on Zermatt.

I agree that the Berner Oberland beats Zermatt for variety, price, and the expanse of beautiful alps (for instance, the 360-degree view from the top of Schilthorn). I also wouldn't travel 5 hours roundtrip in a day to see it (3 connections each way, plus time and connections on the mountain lifts when you get there). I was going to say maybe if the weather is perfect, but if it is, you'll still find many options closer to your home-base, such as a romantic hike with cows, and/or the train to the glacier at Junfraujoch (11,000 ft elevation).

The key with Zermatt... the steep incline of the lower slopes and the geographic positioning of most of the peaks means that from your vantage point in the valley, the scenery is less impressive (other than what you can see of the Matterhorn and Monte Rosa). But once you reach a certain elevation above the town, and with your line of site freed from the blocking views of the valley, the vision of the jagged snow-capped peaks towering high above you is more stunning than anything I have seen elsewhere in the high mountains of Europe. (and I've been all over the Alps). And yes, I think it blows even the Berner Oberland away. But you have to make a consider physical effort to reach this point.

But I agree with the others. Due to the vagaries of Alpine weather, going for only one day runs too high a risk of getting rained or fogged out of those majestic vistas. If you decide to include Zermatt, give it a minimum of 3 nights.

Agree with the other posters that Zermatt isn't about Zermatt at all...the town is uber-touristy and uber-pricey! If you decide to go, immediately bypass the town and get on one of the lifts - I prefer the one that goes up to Schwarzee and Glacier Paradise. The Schwarzee station is right next to the Matterhorn and I think it's quite an amazing view. There is a hotel/refugio there that has a great outdoor deck where you can have a drink or a meal while enjoying the view. Continue on up to the top, Glacier Paradise (breathtaking gondola ride), and go to the uppermost lookout deck. It is one of the more spectacular views in Switzerland in my humble opinion!