I've never done this, but YES it should be entirely possible. The Xcode instructions set it up for you to have a specific template with all of the phonegap files already in the proper locations. So it's easy to get going. But it doesn't do anything else that is particularly special. Theoretically you could simply setup all of the files in a theos project.
To do this you will need to 'reconstruct' exactly what goes into a phonegap template. I have no experience with theos, but a rough idea of what you will need to be able to do includes the following:
- Import PhoneGap.framework
- Reconstruct AppDelegate code
- Import Supporting Files (PhoneGap.plist)
- Additionally you will need to configure the folder structure with the www folder (that the webview loads from) including the index.html file and the phonegap-1.0.0.js file.
So yes, it should be possible. Let me know if you are successful. Good luck!