Human reproduction is a complex dance, a gamete cell splits producing two complete copies , then each of these cells splits again, with half of the DNA moving to one side, and half to the other. At this point the DNA is shuffled around a bit, producing two partial complements of DNA, the cell splits, and we get 4 eggs, or 4 sperm.
When a sperm meets an egg, it's 50% complement of DNA is matched with the egg's complement, giving us a new, genetically unique individual.
Of course, this DNA is useless without the specific machinery to read it. If I gave you a flash drive containing a bunch of encrypted data, but you had no decryption software to read it with, or even a computer, or even electricity, that data would be worthless to you.
A human cell is a vast factory able to read data from the DNA and convert it to proteins which then fold in very specific ways to create enzymes. Seriously, it's awesome. Use different machinery and you'll get junk, just a mess of dead protein that doesn't do anything.
Moreover, the proteins produced are themselves molecular machines, or components thereof, they probably would not have the same effect. Imagine putting a fast washing machine motor inside a laptop and expecting it to speed up the processor. It's just not the same piece of kit, the components can't work together like that.
Even more than this, these molecular machines need to have many other equally complicated machines around them in order to work. Imagine putting a pie machine in the middle of a car assembly plant. It wouldn't do anything, there are no raw ingredients being supplied to it, there's no oven to bake the produce, no conveyor to move the pies to the packaging facility, no delivery lorries waiting outside. It just wouldn't do anything.
So we have a problem. Since standard terran sexual fertilisation requires two exactly matched complementary sets of DNA, and a very specific set of molecular machinery and decoding equipment to correctly read and interpret the data, and then a very specific environment in which the molecular machines you've produced can interoperate, it would be difficult for an alien to procreate with a human in the standard way.
This doesn't mean it's impossible though.
DNA is expressed within a human, the human itself is a DNA decoding machine. Perhaps the alien can make use of the human's own expression of it's DNA.
Perhaps information is transferred in some other way unknown to us. Some holographic information is collected, allowing the alien to mimic the structure and form of the human without directly reading the DNA as with Ridley Scott's Alien, which mimics the structure and form of it's original host to gain a survival advantage.
Perhaps the alien has the ability to mimic the machinery of a human cell somehow, maybe by absorbing a complete cell, or by absorbing two complementary cells creating an embryo out of human tissue and then manipulating it somehow to produce a hybrid.
Perhaps the Alien is able to modify the development of a foetus, changing it's gene expression in clever ways.
Perhaps it taps into some other form of information exchange unknown to us.
Perhaps it's only interested in some specific feature of our cellular anatomy, such as our ability to metabolise glucose, or the general structure of our brain.