Do you have the JamPacks installed?
In the Jam Pack "Voices" is a Software Instrument "Synth Soprano Solo". That is the closest you can get to a female solo voice with the ready made instruments.
The JamPacks are available as part of MainStage 3 bought from the AppStore.

Or create your own vocal instrument by duplicating a "Sound effectss" Instrument and adding your own samples to the scales.
- Sing the scales once and correct the pitch with flextime to get it perfect. Export the scales as an audio file and reimport it.
- Cut out regions "C", "C#","D". etc.
- Create a software instrument track.and set it to "Sound Effects Instument". Safe the Instrument with anew name . Now modify the sound effects by dragging your samples to the corresponding keys and you can play your own voice perfectly in tune.
- Safe your new instrument.
See this help text: GarageBand '11: Play sounds with the Sound Effects Software Instrument
You will see the sound effects you added to your custom instrument, when you hover the mouse over the keys. This will display the name of the sample the key will play.

Regards
Léonie