extend parts in an existing audio file to make it sound longer without stretching the file
Sounds for me, like non-linear time stretching can accomplish what you want. In your example, you would apply a stretching function with a slope of one most of the time and for the single note (you want to be played very long) the slope then needs to be strongly different from one.

Overview paper: Jonathan Driedger and Meinard Müller. A Review on Time-Scale Modification of Music Signals, Applied Sciences, 6(2): 57–82, 2016. [DOI]

Code: https://www.audiolabs-erlangen.de/resources/MIR/TSMtoolbox