As Alec mentioned in the post below, those are the files you need to change.http://pkp.sfu.ca/support/forum/viewtopic.php?f=9&t=3034&p=10689#p9820
And for example, in this file: classes/submission/form/MetadataForm.inc.php
You will need to add a field in readInputData() function to read what the user entered. Then in the execute() function, you can use something like the line below to access the read data and set it using the articleDAO functions:
Of course you would also have to add a function in Article class file and the DAO file to get and set the new field.
I hope this can get you started.