You can consider:
1. Using EditReady to make a .mov with a different codec such as ProRes. AMA link to that. If in 8.3, you can go to source settings on that clip(s) and set it to project rate rather than clip rate creating a frame for frame slomo.
2. Drastic Technologies makes an AMA plug-in that supports XAVC-S and a single codec plug-in will run you $99
3. Create a new clip with Avid Resolve setting timing to playback rate before creating new clip.
XAVC-S has been a point of frustration for a while now with MC owners… and users end up between the cracks with Avid pointing to the camera manufacturer to support, while the manufacturer decides what it wants to support or not. Meanwhile, other NLE’s just support it.
Michael