Question: If a malware can insert from the clipboard a password to a webform in a webbrowser, than can it "in general" insert key presses too?
Related to password fields, that disabled the clipboard CTRL+C/V input via a password manager.
Yes, a virus can do nearly anything it likes, including capturing or simulating key presses and mouse movements.
However some technology tends to make it harder under some conditions. For instance on Windows the Ctrl+Alt+Del key sequence is handled in a very specific way, moreover some application take advantage of Windows' Secure Desktop feature to enter private information like passwords which is precisely designed to keep any other user's processes from interacting with this secure input.
However, since at the end any input is handled by some software, a malware advanced enough which managed to get the highest privilege on your machine (the SYSTEM account) will be in measure to bypass even these protection.