My Slices to Pattern tool can help you out here.
It can take a sliced sample, then automatically place all the notes into the pattern for you, with all the correct note delays to maintain the original timing/groove.
The only important thing you need to do is set the correct Beatsync value on the first/original sample in the sliced instrument, since the timing of all slices will be based on this value.
The tool is not yet fully updated for Renoise 3.0, but it will automatically upgrade itself during install and should still work in the Pattern Editor.