Ableton Forum

I'm trying to extend the length of some of the notes in a midi sequence because they are too short and are ending too abruptly, not allowing for a natural sounding sustain to end.

I need to drag them out for a more natural sustain going into the next note, so I need the note before to actually overlap the next note.

When I do this in Live, the note I'm extending won't simply extend and sit on top of the next note, it eats up the next note, making it go away completely.

What setting do I have to change to remedy this?

Try to increase the Release fonction of your amp enveloppe whatever softsynth it is

Is there a way to apply sustain after the fact, to a midi sequence, or would I have had to recorded it with a pedal initially?

I'm not sure I understand correctly, but try decreasing voices/polyphony to 1, and then add some glide or portamento. Not all plugins have these settings though.

Can you change the envelope of your instrument? Use delay?
What about the Note Length plug-in (in MIDI effects)?

cc# 64 usually controls the sustain message for synths. Other than that you really need to provide more info on the system.