In the Star Trek: Voyager episode "In the Flesh", Species 8472 built a training facility with a simulation of Starfleet Command & parts of San francisco. There they genetically modified themselves to appear as members several Starfleet species. My question is, if there genetic aterations were as extremely sophisticated as the Doctor claimed than could they reproduce & have babies, either amongst themselve or with members of the species they are imitating. Also, if they could would that child be Species 8472, some kind of hybrid (maybe like Spock or B'Elanna) or, for example, would the child be a "full blooded member" of the species that was being imitated ?
The change of the Species 8472 imposters as seen in Voyager were not true changes to humanoid form. They were reshaping themselves structurally to emulate humans and fool casual bio scans. Unless they have some way of disabling their immune system @Hiram is correct, it is quite active in attacking organisms outside the body.
In "Scorpion 1" Voy s3x26 we see that not only is the biology of Species 8372 able to defend against borg nanites it also will attack other species as seen with the dead borg and the infection after Harry comes in contact on the disabled cube. Also, their biogenic field makes them nearly invisible to sensors and can prevent transporter lock. It is very possible that it can be repurposed to return face biosigns to sensors.
In "In the Flesh" Voy s5x04, we see that their transformation is not externally applied, but an internal process of restructuring their biology and their biogenic field to look and scan as and Federation race. As individuals lose control of themselves they revert back to their true forms. Also, the changes were not to DNA as a simple dna test, not a scan, was able to reveal the imposters.
During "Someone to Watch Over Me" Voy s5x22 the Doctor remarks "Species 8472 appears to have as many as five sexes." This would indicate that the cultural exchange at the end of "In the Flesh" included their mating process. It also shows that they require five different gametes to produce offspring.
Between the lack of necessary gametes, the species's incredibly proactive immune response, and that their alterations are literally only skin deep it is unlikely that a crossbreeding would succeed without major intervention. As well as their view of our galaxy as a "threat to their genetic integrity" it is unlikely a crossbreeding would be attempted willingly by them.
Species 8472 would not be able to produce in human form, as they mentioned in another episode 8472 has 5 genders, and when they take human form, they only have 2.
No they would not be able to produce a hybrid. If they had intercourse with a women the women would become infected by their cells. If they had intercourse with a male, their body would kill off all of the seaman . There immune systems are completely resistant to outside forces which is why the borg cant assimilate them.The borg talk about not being able to assimilate them in s04e01 Scorpian part 2. If there genetic material enters a foreign host it proceeds to start to kill the host off. This is from the startrek wiki startrek wiki, and wikipedia. This is both in reference to voyager as well as STO. They also have 5 sexes, as well as not being genetically altered from the outside they are able to do all genetic alterations on command internally. We see them attack in voyager with their claws as well leaving infection behind in the victim which is further proof that their genetic material is Poisonous and attempts to kill anyone infected. (hence why having sex with one to produce a hybrid is a bad idea. )