Im not sure there is a great way to extend a cymbal like that.
You could try stretching the audio item by alt+left dragging the edge. The cursor will turn into hand when you’re in the right spot. With “preserve pitch…” on, it is literally resynthesis, so likely to create artifacts which will be more noticeable the further you stretch it. You might try changing the item’s resample mode to see if any are better for what you’re trying to do. With preserve pitch off, it will just pitch down, not creating those artifacts, but also maybe not being exactly what you want. I can’t say for sure if you would need to “commit” those changes to the item (easiest to just glue the thing), or if they will automatically reflect in RS5K.
But I do wonder if you might get a little closer by manipulating the volume over time. Like rather than try to stretch the tail, make it stay louder longer with like an envelope that ramps up as the wave tries to fall down. If you go too far with that, you’ll start to hear the noise/dither/quantization error in the sample, but it might buy you just enough? Again, not sure if you’d need to flatten that for RS5K.