Here is my workflow to create and import own multisamples into the OP-XY:
Creating multisamples
I use Autosampler in Logic for this job. This is the only tool that can automatically loop almost any audio material seemlessly if you use the magical Penrose algorithm. Btw, Autosampler is also included in Mainstage.
If you want convert existing multisamples in other formats I recommend the free ConvertByMoss.
Mapping multisamples
For this task I use the amazing OP-XY Multisample Patch Generator.
Since today this tool reads wave files with embedded loop and rootkey tags and creates automatically a patch file in OP-XY format.
Transfer multisamples to OP-XY
Now download the resulting file, unzip it and transfer the folder containing the multisample patch to the OP-XY preset folder via MTP.
Done 