Can I add a graphics card to a Dell laptop?

No. You can’t add graphics card in your laptop, be it Dell or other brands. If you have a laptop with a decent CPU and a good amount of RAM, you can at most add an external graphics card in your laptop by sacrificing the WLAN card, which is there in your mini PCIe slot. Also, it will increase your gaming performance but it will make the laptop highly non-portable.

The best option is to buy a new laptop with a dedicated GPU, more powerful CPU and more amount of RAM. Now, even the laptops having dedicated GPUs like Nvidia MX250 to RTX 2070 are non-upgradable. This is because the GPUs like MX250 are basically soldered in the motherboard, making it impossible to remove. For desktop GPUs like RTX 2070, there are different components (like Core, Memory, CUDA cores, Shader Cores etc.) soldered in different places in the motherboard.

Only upgradable components are RAM and Storage.

I heard that there are plenty of Wifi-Cards in a USB format though.

You can't install a new graphics card (a.k.a. GPU) into that Dell. You may be able to update your drivers for what it does have.

. TOS76>

You can, however you will need to practically rebuild your Dell from scratch. Still cheaper than buying a new one in my opinion.