Sidecars aren’t imported by themselves.
For every photo and video that PhotoStructure imports, it looks for any relevant sidecar files, and copies those alongside the imported file. Versions prior to v2.1 had a couple defects that could prevent some sidecars from being copied into the library, but I believe I have addressed that issue in v.2.1-alpha.1.
I’ll add this to the info command now: it’ll be in the next build.