the easy way is to use slices at various desirable start positions within the sample and map them as a drum kit in keyzones, using different midi notes to trigger different start positions.
if you want finer control, you could make a set of phrases that each have a different sxx command tied to a single c4 note event (disable phrase looping), tie a macro to phrase number via a doofer that contains a hydra tied to instr macros device phrase number, then tie that to an instrument macro. This would allow a selectable start position via macro, but it would be tedious to set up the phrases (even with duplication).
OR, you could make one phrase that is filled with c4 note events on every line with interpolated sxx values (from s00 to sff beginning to end), set lpb to 1, sample trigger to one-shot, phrase key tracking to offset, and use various midi keys to trigger the sample at different positions. This assumes you want the sample to play as a one-shot
Check out the loop marker automation tool, too: New Tool (3.3): Loop Marker Automation - v1.01 - #32 by Achenar
honestly, it’s stupid that we don’t have an easily automatable way to offset sample start position in renoise, imo. seems like low-hanging fruit to me, but what do i know? 
hope this helps