Hmmm... you might consider for this as well as your other related question, to develop a 'registration' system in which, when a customer buys on Magento, her/his data is updated on Magento from the main/ reference app.
This would allow to smoothly use Magento sale process
You may find in the Magento book from php|architect a good example of similar integration. Their example synchronizes Magento data with a CMS, but it should give you a good vision of all that is involved.
Note: the book uses an "old" version of Magento, but at this design stage that should not be a problem
Remark: since your 2 questions are so close, you might consider closing the others, so that all answers come just on this one. You may do that by going to the other question and "closing it". If you do not fond how to close it, use the "request attention" link to get some help from EE's administrators.