Version: (using KDE 4.2.0) OS: Linux Installed from: Fedora RPMs In Pidgin, there's an option to start a new chat (Buddies -> New Instant Message) that doesn't rely on clicking on a buddy list entry. This is extremely good for chatting with invisible users. Please add a New Instant Message/New Chat option to the File menu that will behave like Pidgin does.
To start with a chat with an invisible or offline user, you could switch to "Show all" and select the user. But I second that this option would be useful, especially if you want to have a short conversation with someone, you don't want to add to your contact list.
Dear user, unfortunately Kopete is no longer maintained. Please migrate to another solution, e.g. for Jabber a possibility is Kaidan, for Matrix a candidate is NeoChat.