Shea if you go to your knuffel's page, not the one where you can feed/play with all of them in a row but click on its picture and go to its page where you assign its points when it levels up...
(sorry I don't know what the official terms are for that)
Anyhow.
Look below the XP bar,
there is information there about changing its name and getting the BBcode, etc.
Right around in there somewhere there should be an "adopt" or "adopt this knuffel" button choice.
If you click on it, you can pay the food points to adopt it permanently.
You can also adopt one just like it from knuffeltropolis and have a second trial knuffel of the same kind, but of course your first trial one presumably has a head start in leveling up, so it's nice to be able to keep it.
(I lost mine because I didn't know)
If you need

to adopt it, a couple of the charities will help you out!