They can, and they are - we planted one in the backyard when we came back from our first family trip to the USA in 1979. It is now some 30 meters high and dominates the area, the iron fence needed to be moved so the trunk (which is about 1.50m in diameter at chest height) fits, it is encroaching a bit on the cycle path behind the house. This is in Abcoude in the Netherlands, a village just below Amsterdam. There are several specimens of Sequoiadendron Giganteum [1] in the Netherlands, among them one [2] in the arboretum at Wageningen University (where I studied forestry). It is a hardy species which does quite well in the wet Dutch climate.

[1] https://en.wikipedia.org/wiki/Sequoiadendron_giganteum

[2] https://www.monumentaltrees.com/en/nld/gelderland/wageningen...

In principle, yes. But wild land is being encroached upon worldwide. And other stuff is already trying to live on what is left.

When the seas rose from 20kya until 8kya, inundating millions of square miles of prime habitat, people moved uphill. But other people had already been living there for millennnia. Often those were peoples who had been driven out of prime bottom land, and were making do in marginal circumstances. Guess what happened then.

Giant Sequoias typically grow between 4000' - 8000' feet. I've been growing redwoods for the past two years and have 20+ Coastal Redwoods, but most of my Giant Sequoias die from being so low to sea level.

https://www.savetheredwoods.org/redwoods/giant-sequoias/

They already are, but it doesn't make natural forests of them. A few weeks ago we had a discussion about a database of sequoias in British Isles: https://news.ycombinator.com/item?id=28047458

It's possible. I know a place in a remote part of France with one sequoia. I don't know how it arrived there (there is nothing of interest). However is it a good idea to massively plant sequoias outside their native range ?