Why would that be an impediment? As far as I understand it, the principle of voluntary exchange relates to buyers and sellers in a market, not to the commodity in which the market is made. The moral enormity inherent in chattel slavery, namely that it reduces human beings to a commodity in which a market is then made, has nothing that I can see to do with whether such a market can operate in an economy run on capitalistic principles.
After reconsidering my statement, I think you may be correct. Slavery can be capitalistic if you only include the buyers and sellers as members of the market.
"Slavery is capitalism" is most certainly wrong though.
"Capitalism encourages slavery in plantation economies" would be a better argument, but might also be refuted given the gradual abolishing of slavery despite the continuance of capitalism.
The fact that slaves are not free means that exchanges couldn't possibly be voluntary.