Hey Benoit,
I'm with JM. I've used 1Password for six or seven years now. The price made me balk at first, but after I'd used it for two days I realized it saved me so many headaches it was worth the money.
KM itself is probably not the ideal vehicle for creating strong passwords, but you might be able to build a pretty good macro if you try.
Scope out:
Google:
os x generate strong password
KM can access the OSX Keychain (see the wiki and search for 'keychain'), so if you're determined to use it many things can be accomplished with a little (or a lot of) effort.
Even so — I recommend a good purpose-built password manager.
-Chris