keychainpassword
When signing into Skype, the password fields don't offer the Keychain password-generation option. Is there anyway to generate a password and store it manually in Keychain?
I need keychain to remember it, as there's no way I'll remember QW45FX-ASDASD style passwords, and MS won't let me re-use all my memorable phrases, as they've been passwords previously.
Are you sure you aren't using ssh-add from another source? On my Mavs, ssh-add—which is /usr/bin/ssh-add—does have the -K flag. It's in the manpage, shows in the usage message, and is accepted.
Check which ssh-add to see if you're pulling in ssh-add from elsewhere due to your PATH.
To get the password from your keychain . You can right click on that item of which you need the password and click on "copy the password to clipboard", then you will ask to enter the login password , after that you can paste the password where you want. 
PFA:
Best Answer
For apps like that, not only will it not offer a suggestion, it probably won't remember whatever you put in it manually.
For things like this, I either try to use their web-page equivalent, which at least will get it into Keychain so I can look it up manually next time, or if there is no web option, you can make keychain generate passwords manually, though you still have to do this manual search for it each time, then copy/paste to the app…
From Keychain Access, + button or Cmd ⌘ N
Give it a memorable name, account name & then click the key button lower right.
You can choose length & password type.
Once completed click Add.
Alternatively, if you also have an iOS device with the same app & password if you fill it in there manually, then Keychain will remember it, but only auto-fill it for iOS, not for the Mac app.
File this under irritating. ;)
Late thought - watch out if you get iOS to add it to keychain after doing this manually from macOS, because you will likely end up with two independent entries.