ScaleMate offers quick access to the built-in instrument scales in Renoise, organized by the number of notes.
If the user-interface is visible, the currently set scale and key are shown there. Push a button to select.
Alternatively, use the available keyboard shortcuts / MIDI mappings.
Whenever you select a scale or key, you can tell ScaleMate to write this into the pattern (technically, a MIDI command).
This also works while recording, and allows you to switch scales on the fly.