Hi and welcome to Arturia forums.
I was led to believe custom samples could be imported. Is that wrong?
No it's not wrong.
Click the folder icon on a instrument slot and browse to the soundfile you wan't to load.
Not mentioned in user manual
Have a look in the manual section "5.1. Instrument Slots". There is further informations and visuals.
But beside that, then some things about what's avaible in CMI's own sample browser, and what's written in the manual about this sample browser can clearly be better.
(It can also be a good idea, to browse and search the forum to get informations. The information you need is allready availble through the latest thread - just before you posted your thread.)