If you set the field to allow rich text formatting, you can set the richValue property of the field to include spans, which can be used to specify strikethrough text: http://livedocs.adobe.com/acrobat_sdk/9.1/Acrobat9_1_HTMLHelp/JS_API_A croJS.88.721.html
The sample scripts shows how to set up the field for rich text formatting and set the richValue using spans. If you get stuck, can you give an example of what you'd like to set the field to? For example:
Here is an example of some sample text