Sorry to make this specific
I have X - beyond the frontier on steam and want to buy X-tension which is now on GOG

Unfortunately you can't buy a DLC on GOG without owning the base game.

If the add-on is a standalone game, then you can buy it on GOG. However, if it needs the base game, then you need to buy it on Steam.

However, in the case of X-tension, GOG doesn't even seem to sell it separately at all, so you can't buy it either way. GOG only sells "X: Gold" which includes both X: Beyond The Frontier + X: Tension.

So if you want X: Tension on GOG, you need to buy X: Gold. Sorry but that's the way it is, no way around, you will just have to accept it, such is life in a big city.