Query String, Custom Fields, and XML reporting

Hi! Absolute Newb here! I'm not sure if this is the correct location to post this, but it seemed the most appropriate. The company I work for is interested in switching our shopping cart to Magento and I had a few questions.

I'll start with these 3:

1 - I work for a biography publisher and we use email promotions to drive our listees to our marketing site and promotional pages. Each listee had a unique ID that is passed through the query string. I was wondering if a value could be extracted from a query string and placed into a hidden field to be passed along to the checkout process. I was also wondering if the same could be done for a promo code passed along in the same way. I don't know if Magento URLs are firm, but can Magento fish out values from a URL string that is linked to a Magento site/cart? Which leads me to...

2 - How easy is it to create custom fields in Magento? I am referring to the question of listee IDs and promo codes above being placed into a hidden field and then passed along though the checkout process. Does this sort of thing require a lot of behind the scenes coding or workarounds?

3 - We already have a fulfillment system and we run reports by extracting the data to an XML file. Is it possible to use XML or some other flat file to insert data into our fulfillment system? In other words, can Magento generate XML reports?

A lot of stuff here, thanks for your help!