How to add 18.04 repo to 20.04?

Solution 1:

Technically it is possible to add bionic source list. You can edit /etc/apt/sources.list file for it and replace focal with bionic for main repositories. I would not recommend this way as it will result in broken dependency tree.

To work around you can try downloading .deb package from Bionic repo. Here again it will not work without its dependencies and libraries may be conflict. Try it on your own risk. This is also not recommended way. You need to wait until bug is fixed for Ubuntu 20.04 as of now.

Solution 2:

It might be as simple as adding the line deb http://de.archive.ubuntu.com/ubuntu bionic-updates main to /etc/apt/sources.list.d/bionic.list, installing the bionic package and then using pinning or some other technique to prevent the reinstallation of the focal package.

If there's too much technical jargon in those sentences then it might be advisable not to try and pursue that route. Quite certainly it can result in a messed up system unless you understand what you are doing.