Renoise has no timestretch. I don’t think any of the tools (Rubberband and Akaizer) are realtime. Afraid you might be out of luck. Unless you were to program them into patterns as slices hits, then you could change the tempo of the playing patterns and each slice/sample would play at the original pitch (you would lose tails or gain bits of silence due to difference in tempo as it does not stretching though. A bit of reverb can help to hide silent gaps.) There is a tool which will put all slices precisely where they should be in the pattern, which might help you if you wanted to keep a groove from them.