As vonbrand told, you should never use that kind of chars in a password that you are expected to type in a console !
Console is the last ressort thing.
Now, to answer your question:
First, you must ensure the console is in utf-8 mode if you want to input utf-8 (otherwise, you have to ressort to input of individual bytes of utf-8 sequences, ouch!), and you need also to have a font loaded that shows those characters (for basic latin letters it shouldn't be a problem however).
You can input it 3 ways:
* if you have a mouse and gpm running you can copy and paste them (write the login greeter message so that is display all unicode chars you need)
have the need unicode symbols available in your console keyboard (eg: "U+00FC" for lowercase "ü")
have a dead_diaeresis (and/or any other needed dead keys) symbol in your console keyboard, and a compose file definition.
note however that if it hasn't changed from last time, the compose file use 8bit, it is converted to unicode in an ugly way; it only works when result is in the range 0x0001 to 0x00ff (which is the case of "ü")
Note however that chances are high the right keyboard map is already there; just maybe you are loading the wrong one.