Is it possible to glue floating parquet flooring afterwards?

That doesn't contradict each other. If you do it yourself, you have to be prepared to possibly pay your dues or make compromises in quality. You are doing that without complaining, so all good.
Respect for the skill of the experts is definitely appropriate. They exist among craftsmen and also among hobbyists.

I do that. I have respect for the craftsmen and their work. But there are things that you as a hobbyist are more likely to take on yourself. Laying parquet is one of them. And it's no problem if it's not perfect, only the creaking is really annoying.
Now I have several answers here... from leaving it be to trying it out.
Let's see what I ultimately do. I wanted to get some information first if it would generally be possible. Thanks

Basically, parquet is a natural product. And every parquet floor will creak. and it does. Even if you glue it down. However, it is also a quality feature. It simply belongs to it. Wood has to work. Summer and winter. Humid and dry. It does not matter which parquet you choose.

The carrier layer or underlay is almost always spruce and fir. Above that, the wear layer is made of high-quality woods. Because both work differently, the parquet creaks. If parquet is glued down, then only solid wood parquet without a carrier layer.

So basically, parquet is a natural product. And every parquet floor will creak. and it does. Even if you glue it down. However, it is also a quality feature. It simply belongs to it. Wood has to work. Summer and winter. Wet and dry. It does not matter which parquet you choose.

The carrier layer or base is almost always spruce and fir. Above that, the wear layer is made of high-quality woods. Because both work differently, the parquet creaks. If parquet is glued down, then only solid wood parquet without a carrier layer.

It depends on how much it creaks and at how many spots. In the hallway with almost every step, I find that annoying.
In old apartments, such creaking belongs for me, in "new builds" or renovated houses, not for me.
Can only solid wood parquet be glued?

I have laid about 120 sqm of Doussie parquet throughout the house. A very beautiful, warm, but also hard wood. The support layer is spruce wood with tongue and groove. Partly long pieces with 2.00 m or 1.20 m length. Installed floating. It creaks now and then. But it always gives me a feeling of the old villa. I like it and wouldn’t want to miss it. However, I would not glue it, since both the support layer and the wear layer are likely to move relative to each other.