Can there be error if I change the list file of ppa and replace codename with focal even if I am on impish

If the program works, and apt does not throw an error while installing other programs, then everything is fine.

However, in general, there can be complex dependency issues with some repositories.

For some general advice, see the comment by @sudodus.

Suppose, the program (say, A) you install also installs a dependency, B, which is available in the PPA. However, suppose you have another package (C) installed, which has a conflicting library with B (but the version of C in 20.04 did not have this conflicting library). Then, apt would need to remove C to install A. Further suppose that the GNOME Shell depends on C. Then, while installing program A, apt will ask you if you want to remove C, gnome-shell, along with a bunch of things. If you agree without carefully reading, you will be greeted with a black screen after the next reboot.

Such things actually happen. Someone carelessly tried to install Steam in Pop OS, which removed the GUI.

I can see in the package details that the same package is copied over between different versions of Ubuntu, so it should work fine as long as all the dependencies are available. I have emailed the maintainer to create a package for Hirsute and Impish.

However, it would be safer to simply download and install the .deb package than adding the focal repository (just in case there are any future revisions, which works in focal, but causes dependency issues in impish).