The title is pretty self-explanatory, but what I mean is: if you play the C8 note, for example, and keep your finger pressed to play it for a while, it would sound as if you'd just pressed it for a small period of time instead of holding it down. I suppose that is because the sound dies down really fast. So my question is if there is a way to sustain the sound for longer than it usually does.
You've accurately described what happens when you play a high note on a piano. That is what a piano does. You could of course capture the sound and prolong it with an external device. Record and loop it. Apply heavy compression (but watch out for the noise floor rising as it struggles to amplify the rapidly decaying sound). You could excite the string by some method other than the piano's hammer. Might br tricky to get a bow onto it, but perhaps some electro-magnetic device like an EBow? http://www.ebow.com/home.php
You can play harmonics on the piano by lightly touching the string at points that are at integer fractions of the string length, essentially dividing the frequency by that fraction.This is in essence the same technique used in guitar. Obviously it is hard to play anything intricate like this, and often requires some reach. A harmonic at 1/4 of C6 or 1/8 of C5, if well performed, could sustain longer than the fundamental C8, despite being the same pitch. This may or may not be what you want. Video
Not without modifying the piano or adding something like a pickup and amplifier to get feedback. That would be expensive, difficult, and wouldn't work well.
You could get a keypress sensing bar that goes on top of your piano keyboard and detects when you press a key and then sends a MIDI NOTE ON signal with the key number and use that to control and external synth that plays a sustained piano sound until you release the key.
If you want it to be a real piano that is not modified, the closest technique would be to play the note repeating very quickly to create a continuing tone. If you play it quietly then the hammer noise will be reduced and further strikes should increase the volume as you excite the strings more. You could doctor the hammer to make it softer and reduce hammer noise.
This is why some consider the piano to be a percussion instrument. It is not meant to sustain or swell.
The notes F#6 and above on an acoustic piano are undampered anyway, so they ring for the same length of time whether you press and hold the key down or press and release immediately. And as the notes are undampered, use of the sustain pedal will make no difference to the decay time of those highest notes.