Both kdm and (I'm fairly sure) gdm allow a choice of graphical shell (desktop environments and window managers) when logging in. I imagine that this works on openSUSE and any other distro. This menu gets populated from /usr/share/xsessions/. Pretty much any desktop environment or window manager (even twm!) will add a file to that directory, so you should get the choice of just about every desktop environment and window manager you have installed each time you log in. That seems fairly easy already.
Is the problem that you want more flexibility to change graphical shell mid-session? Certainly some things can be changed mid-session: many window managers allow switching to a different window manager without closing unrelated applications. I suppose a question arises as to what a user considers to be "part of the graphical shell": e.g. if starting a certain desktop environment starts a file manager, clipboard manager, auto-mounter etc., then logically those extra processes should be stopped when switching mid-session to a different graphical shell, whereas maybe the user doesn't want their current file views to disappear, or lose other state. I'm not very knowledgeable about such things, so I'll stop here.
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