You can use the roller/looper to roll a note or loop a series of steps in real-time, but you can't record roll or loop into the sequence.
You can change the velocity and gate lengths of all the steps in a sequence at the same time either relative to their current settings or so that they all go to the same setting by pressing and holding shift then changing knob one (for relative changes) or knob two (for absolute changes). Whether you are changing the gate time or the velocity level depends on what mode you are in.
To change all the notes of a sequence relative to their current values, use the sequence transpose function. To change all the notes to the same note, press and hold step one, hit the note you wish to use, then hit the last note. That will do a quick program of all the steps from one to sixteen to the note you hit.
Make sure you save the pattern once you have finished all that or your changes will be lost as soon as you change patterns.