You can however down the extension from the website and install it from the terminal.

codium --install-extension {path to .vsix}

You are able to do so, but is it allowed by the website's terms of service? It may say that you are granted the license to extensions only with Microsoft builds of vscode.

Microsoft isn't a stranger to distribution restrictions and software usage limitations. I remember uploading Visual C# Express 2010 (freely downloaded from Microsoft's website, without license keys) to a local file sharing website to ease the downloading for my local study group and got a letter from Microsoft's lawyer to take it down.

After that our study group transitioned to Mono with Monodevelop.

Terms of service? Who cares.

An actual example is that the Python LSP extension on the offical marketplace has some "DRM" that makes it pop up a fatal "You can't use this extension except with the real VSCode" error message. People have been playing whack-a-mole with it by editing the obfuscated JS to remove that check, or by using an older version from before they added the check. https://github.com/VSCodium/vscodium/discussions/1641

THE PLAN

1. Install the editor version free from proprietary stuff from big corporation

2. Install a proprietary editor extension by big corporation

3. ?????

I don't remember ging to the Website and agreeing to anything. I got vscodium from my package manager.

If you're idealogically opposed to Microsoft's editor, that doesn't seem to be a problem to me.

If you're ideologically opposed to Microsoft $FOO, you want to avoid putting yourself even further into their embrace.