an enzyme called reverse transcriptase or its equivalent is required, the HIV virus and a number of other retrovirii do this, and implant thier code into the host DNA.

eukaryotes use splicing mechanisms that copy/delete/replicate DNA from one locus to another, this is more reliable as mRNA are unstable and become damaged quickly. cytoplasmic mRNA is not of fidelity and does not reverse compile into the dna sourcecode, or it would produce massive errors and corrupt the DNA.