In all honestly, it's best practice to apply a quarantine for all new ferrets for 2 weeks long. They should be kept in a separate cage, preferably in a separate room. This is used to prevent nasty things like ECE from happening in your ferrets. It also allows the new ferret time to get to know you and become a little more used to their new environment, albeit a single room for the time. And it gives you time to get them started on the correct diet. They should also have their vet visit done ASAP as well.
A 5 month-old kit should take to raw straight away. I brought Ares and Apollo home as 7 month-old kits and they got started on raw the second they stepped into my home.
If your ferrets have already met one another, it may be too late to quarantine, so please keep an eye on everyone's poops. You do not want to see green ones. You may also opt to giving the new girl and your others some Rescue Remedy to both help still her anxiety over a new home, as well as theirs over a new addition to their business.
Epizootic catarrhal enteritis (ECE) is a highly contagious viral infection in ferrets. It often recognised by the inflammation it causes in the ferret's intestines. Older ferrets develop the severest form of the viral infection.
I would suggest reading this article. Having dealt with ECE a number of times and having lost a boy to the long term issues of ECE being educated is the biggest part of combating and dealing with this disease
Even a ferret with a clean bill of health from the vet can carry ECE, which is why we do the quarantine.
As for the raw, yep, I ended up giving them some raw soup and raw slivers of pork (with a little bonemeal powder) on their very first day. And once that was accepted (which it was that very evening) I just continued to up the game to chunks and bone-in raw till I got them fully onto my normal weekly raw menu.
She can be a little intimidated by the new environment and people right now, and that does damper appetites a little at times, but you can keep trying to offer her some raw. Have you tried offering her a raw sliver from your hand? Or perhaps a raw soupie? You can apply Dab'n'Grab method with the soupie. You can also rub the sliver of meat against her mouth a little so she ends up tasting some of it. I wouldn't just throw away the kibble she is eating right now until she starts eating some raw by herself though. But you can take the kibble away and offer her the raw and see if she'll eat some when hungry. Remember to have some hours free between kibble and raw so she doesn't get a stomach ache or risk intestinal bacterial overgrowth.
I'd try offering a muscle meat first if you want to do slivers (chicken and pork are quite mild), or the actual soup recipe from on here for a balanced soupie. Organs by themselves can be a bit strong for a ferret first-time round when totally new to raw, so it's best to try with muscle meat first.
Remember, when offering slivers or chunks of muscle meat as meals to a ferret that does not yet eat any bone-in meals, add 1/2 - 3/4 tsp of bonemeal powder per 10oz of meat.