Continuing or streching an analog sound that is too short

You could try several things.  The first I'd try would be a delay with a very short delay time and a substantial amount of feedback.  You'd also probably need to automate the level to get the continued sound to start and to die out at the desired time.  You could also try a very wet plate reverb.  If you have Valhala Plate, that might do it nicely.  Again, automation is your friend.

I've been faced with that exact same scenario. I had a thick four-part harmony singing an "AH" that ended a song, but was about 1.5 seconds too short. No way I was going to re-record four vocal tracks just to stretch that final note.
 
Try this...duplicate the clip in another track and use slip-edits to trim the dupe down so it contains no identifiable transients. Then split the original clip, slide the two halves apart and insert your duplicated clip between them. Slip-edits should be able to make them merge seamlessly, although it'll probably require some trial-and-error.
 
As a final coating of spackle to further hide the seams, apply a thick hall reverb. This can be automated so it only comes in when needed.
 

bitflipper
I've been faced with that exact same scenario. I had a thick four-part harmony singing an "AH" that ended a song, but was about 1.5 seconds too short. No way I was going to re-record four vocal tracks just to stretch that final note.
 
Try this...duplicate the clip in another track and use slip-edits to trim the dupe down so it contains no identifiable transients. Then split the original clip, slide the two halves apart and insert your duplicated clip between them. Slip-edits should be able to make them merge seamlessly, although it'll probably require some trial-and-error.
 
As a final coating of spackle to further hide the seams, apply a thick hall reverb. This can be automated so it only comes in when needed.
 

Why, Dave ... I think you've just invented granular synthesis.