Viruses use the host's machinery to make copies of themselves. However, some human viruses require a type of replication that humans do not normally have. For example, humans normally do not have the ability to convert RNA into DNA. How can these types of viruses infect humans, when human cells cannot perform a particular role that the virus requires

The virus has its own genome that code for any specialized enzymes that virus need that the host does not have.

Explanation:

Virus do not replicate without host. They use the host's machinery to replicate their genome and make copies of themselves. The genome of virus contains genes for their own enzymes, like reverse transcriptase and protease. So virus infect human cell by injecting their genome into human cell and replicate their genetic material by using host's machinery to make all required enzymes. The virus has its own genome and contain genes that code for any specialized enzymes that virus need that the host does not have.