It is hindsight that allows one to state that imported slaves are immigrants. Of course in the period, they were not immigrants as they were not to be citizens (or even considered to be people in some cases) and were only property. They were on the same level as livestock.
This isn't that outdated of a situation... are those who are brought into the country to be sex slaves also considered immigrants? Does immigration require that the person has come to the country of their own free will? If said person arrives here as property and against their free will, wouldn't they then be freed and returned to their country of origin, or if they are made to stay, what exactly would that be called? I don't think "Forced Migration" is a term that fits that situation.