I have been playing Warhammer 40k for over a decade but am not a complete authority.
Yes, a Space Marine can remove his armour according to the Deathwatch core rulebook.
The Deathwatch core rulebook's armour section explains that it takes around 30 minutes to remove or put on power armour with 3 chapter serfs (slaves). In an emergency it can be done in 10 minutes with one serf, but no less than that, as the proper rites must be observed.
I ran a Deathwatch game on a ship where they were not in their armour and had to either put it on or fight without it - very fun and lots of RP opportunities.
In the third Grey Knights novel, the main character is captured by Chaos and made to fight on an arena world. His armour is taken and the book deals with him adapting to not having it.
Do note that when he gets it back near the end of the book, he carries it around, refusing to wear it until it can be purified by the tech-adepts. When he is forced to wear the storm bolter gauntlet it burns his skin.
Space Marines are trained to maintain a state of readiness so would remain in their armour at all times unless there was a point to removing it such as repair.
Space Marines have many enhancements such as extra organs and equipment built into their armour to deal with anything not related to killing the emperor's enemies, such as cleaning, eating or sleeping. They can do their activities in armour, they just don't have to.
As an aside: Space Marines don't have sex - they're eunuchs. Being genetically engineered to kill, genitals aren't necessary.
TLDR:
Yes armour can be removed, check deathwatch core rulebook.
Armour is taken with weapons when captured but is very difficult to get back on in this situation.
Best Answer
Space marines carry out constant trials to find the best of the best on dozens of worlds, those that pass the first test are taken to the home worlds and further tested to make sure no one got through with just a passing grade.
Form there it takes one year to produce a space marine using lost technology, all of which the candidate spends unconscious as multiple new organs are installed and there muscles artificially augmented. This process has around a 20% failure rate.
Your willing host would probably die about 30 seconds into the process.
The gene seed is a control organ, it regulates the rest of the implants and allows the space marine to integrate into his armor wearing it as a second skin.
You are missing the second heart, the third lung, anti toxin glands, toxin sacks for the jaw (yes a space marine can spit acid like alien), brain hemisphere separation and a dozen others. If you are interested the Deathwatch core rule book goes into great detail on how a space marine is made, what is added and what it does.
I doubt you could successfully harvest all of this from the dead marine or the gene seed for the matter. It is incredibly hard to harvest a gene seed, even space marines need specialized equipment for a 50/50 chance of success.
Also consider a gene seed is to a space marine chapter what a piece of the true cross is to a devout Christian, and you have a Deathwatch gene seed. I would not be surprised if the planet you were on simply explodes if word gets out; the inquisition is funny about their secrets.