Possibly I'm wrong, but if you edit a page, mark the lines you want to turn into code snipped then you can just use one of the quickbuttons "TT". Please check
Toolbar for description.
If that is not sufficient you may use a plugin like
xssnipper. That delivers further code-buttons to this toolbar.
If you are not interested into xssnipper features you might want to use a button plugin and define the substitution yourself.
e.g.
codebutton_ext or search the others (e.g. codebutton2, custombuttons)