MIDI preferences -> Record and Play Filter -> Velocities
Or do you mean exclusively by a shortcut?
Edit: if so, the Duplex keyboard might be of interest - I just changed how one of it’s features work, to accommodate this sort of thing.
Now you can choose to sync it with the keyboard velocity in Renoise (the setting in the pattern editor’s toolbar), and every note you enter via the MIDI keyboard will receive a fixed velocity.
Before it was scaling the note velocity relative to the value being input, but I think your usage case is more relevant.