If I want to use Putty directly, I just put putty.exe in KeePass' folder, and then use ssh:// links from within KeePass.
Obviously that's only good when each program takes 1 file. When you use Putty Portable, and especially if you also use KeePass Portable, that's not the case anymore.
You can use cmd:// in the URL field, instead of ssh:// - Then include a relative path to portableputty.exe. See the KeePass help on this for more info.
I've tested and it works.
So, for example if you want to connect to host myhost your URL field would look something like :
cmd://..\PortablePutty\portableputty.exe myhost
The above assumes the folder structure is something like :
U:\PortablePutty
U:\KeePassPortable
You'd have to adjust it accordingly. And I'm assuming the path is relative from the KeePass executable, not from the keepass database ( if they are in different folders ). In my case I only tested with the keepass DB is in the same folder as the executable.