A script for Mac OS x

I need to write the script which's end result is to add one line of text to existing text file. Here is the process road. 1. User lunches the script 2. Choose from the pop up window necessary application (would be grate if the script could be executed via drag drop of application over the script). 3. Scripts runs and searches in package's content needed file (on all systems file will be read only) 4. Founds file, ads needed line of text. 5. Saves the file. 6. Ends execution . Would like to have the source code as well, as line of text to add and file for different users will be different.