Two things.

Someone other than me remembers it exists! (blows noisemaker) Okay, that's a bit of an exaggeration. I know at least Lily Winterwood also remembers it exists, because she writes an agent for it, but I've hardly if ever seen it even referenced in recent stories, so I'd been planning on doing something with it once I got Permission, however far away that will be. Maybe we can collaborate at some point.

Well, considering that "he/she/it/they fell through a plot hole" is among the most common reasons for someone or something to wind up in HQ(and actually is going to be the centerpiece for the DMFF story that I'm planning), having a canonical basis for a character suddenly disappearing and reappearing somewhere else provides a justified prompt to take advantage of the trend. So, yes, it would be possible.

As for plausibility of recruitment opposed to extradition, that would depend on a few factors, but just in general, I'd say so long as he's sapient, capable of interacting with other beings and the PPC technology, and reasonably non-hostile, I'd say chances are high they'd let him in. The PPC is perpetually understaffed no matter how many agents it has, after all.
What is he, by the way? I don't know Spiral Knights, so I don't know what the species are, who can become a knight and how, what their power levels are, and the like, so I can't say much about the matter for certain.

meh it looked cool, I put/DOGA on there co I wasn't sure just how many fics there'd be to spork for the DMFF alone. Collabing would be fun.

Well the Spiral Knights, aren't really a named species (if they are they're called Knights or something like that), they're reasonably non-hostile and sapient enough to be in the PPC.

The basis of Spiral Knights is that you start off as an Apprentice of the Spiral Knights who flew through the cosmos on their ship The Skylark, well did. Having crashed on a mysterious world called Cradle they're basically trying to rebuild the Skylark and explore cradle to its fullest annyoing/defeating various monsters, slimes, constructs and ghosts on their way. You rank up dependent on certain 'Rank' missions (for example I'm currently a Knight Elite and I'm on the Rank 6-1 Missions, you start as an Apprentice on Rank 1-1 and the highest atm is a Champion on rank 9-4) In terms of power they don't actually have anything besides their abilities to create the weapons and armour they need. They have a choice of Swords, Guns and Bombs as weapons (only have 2 weapons at a time but in any combination) and various different types of Weapon and Armour that each do different things. The weapons and armour have a level between 0 and 5 stars, the more stars it has the higher rank required to wield it (for example as a Knight Elite I can use up to 3 star items)
http://wiki.spiralknights.com/Main_Page
Storme Hawk