Sure - though you'll all probably giggle at how I did it with my limited resources...
Here's how I did it, though I am sure someone else can find a better / more efficient way of doing it.
You'll need "Audacity" - free sound manipulation program. Find it at
http://audacity.sourceforge.net/
You'll also need something to record with (I used a simple Headset-with-mic)
1. Go to
http://www.oddcast.com/sitepal/ (a site called "Sitepal"
2. Click on "Try Demo"
3. Click on "Record" and choose "Text to Type"
4. (this is the naff bit) flick the mic round to the left ear-piece.
5. Fiddle about on the voice-box of Sitepal's "Demo" and get a voice you like (I tend to stick with "Kate(US)")
6. Type in what you want, then have Audacity handy, click "record" on Audacity, then click "Preview" on the Sitepal site and it will record the voice.
7. After it's done, listen to it through Audacity, check it is ok.
8. Use the "Export to Wav" option in Audacity then you can import it straight into the game.
Its as easy (and improvised) as that!
TIP 1: Chances are the will be a silent gap at the front of your recording. If you want to, select the "blank" area in Audacity and click the "scissors" icon (far left) to cut out the dead sound.
TIP 2: It is going to record pretty quietly. I'd suggest using the "Effect / Amplify" option in Audacity to spice it up a little.
Hope this helps!