Supressors for subguns are dry cans. A little water or oil in the can may make them a bit more quiet, but as soon as it heats up and and evaporates the sound will increase to the same level as a regular dry can. Basically wet cans lose their 'wet' after a few rounds so a subgun can has to sound good dry. Mostly, wet cans are used on handguns where there is far less volume of fire going through, and where the weight of the can is critical to the reliable function and cycling of the firearm. Cans on the MP5k/PDWs are removable. RDTS, however, has made a special version of the MP5k PDW with an integral can and mini-Uzi style side-folding stock, and I suppose there are other custom jobs out there, but HK never designed an integral can for the MP5k as they did for the full size MP5.
You can use either subsonic or regular ammo through any can, but you'll get more noise from the supersonic crack from regular ammo. That said, you should use subsonic through a detachable can. Integral cans usually have numerous ports in the barrel (within the supressor) to bleed off gas (and velocity) so regular ammo can be used with no more noise than subsonic ammo. The downside is the ports get dirty, clogged and are a pain to clean. Additionally, you don't have the option to remove the can and use regular ammo at normal velocity.