Although there is always a question of planting evidence at a crime scene, I think it is highly unlikely.
Aside from the skill involved one would wonder why such minute amounts rather than a clearly exculpable amount. Simply go to a seedy part of town and pick up a recently discarded item or take some homeless bum home for a free meal, collect some of his blood in a plastic container and make sure his free meal is his last one.

How would there be epithelial cells under the nails and blood in the panties if each had been planted?

If it took them until 1999 to test that second spot would you want to be the person who was waiting for the dna to exonerate him?