Mike said:

Is it possible to uncover the genome of a virus and inject new genes that would include corrective material for damaged cells like cancer and/ or protein codes to target other specific viruses?

Many viral genomes have now been completely sequenced and the use of these viruses as vectors for the transfer of genes is now very common. Adenovirus, retrovirus, herpes simplex virus and many others viruses have been utilized by researchers as vectors for gene transfer. Often the virus is furhter manipulated to make it non-replicating and nonpathogenic.
When it is known that there is a defective or loss of expression of an important gene this type of viral gene therapy has been attempted with successful results. Bubble babies lack a certain gene necessary for the proper development of the immune system. These babies were the first human gene therapy clinical trials where the missing gene was reintroduced into the babies using a retroviral vector.