WordPress front-end, completely custom PHP back-end, doable?

Hey guys,

I am currently running a site that was built from the ground up using primarily PHP. It is a fairly complex CMS that manages all of our memberships, form submissions, advanced reporting, and sales data. As a whole it has performed above and beyond what we had been using previously and I am happy with it. Ironically, the content management portion that handles the front end of our site is very lethargic. I am not able to perform basic CRUD functions on 80% of the site and that is becoming an issue as we continue to expand and add services. At present I am now stuck because there are changes that need to be made that I cannot do on my own and that frustrates me.

In my quest for a solution to this problem I stumbled across WordPress and have entrenched myself in it for the last two days. I have come out with a WordPress site on my localhost that is 95% of what it needs to be. Content management is monumentally superior to what I currently have and the ability to have such finite control over every visual element is exciting.

Having recreated my front end in WordPress with relatively minimal effort, I need to know if it is now feasible to make it talk to my existing PHP back end. The two pieces of functionality I would need to integrate are the membership items (log in, register, view/edit account info, invoices, payment processing) and passing form submissions directly to my existing back end. I have no desire or need to interact with the WordPress back end other then to create/edit website content. However, since this is not done daily, logging in to a secondary system to do this is not a big inconvenience.

This to me seems like it should be doable and would be a more cost effective option then redesigning an entirely new PHP based content manager. Conceptually, if a widget or plugin type item could be created to interface the two key pieces of functionality with my existing CMS that would be perfect.

So, is this feasible or am I just living in la-la-land?
Any input or suggestions on the matter would be greatly appreciated!
TIA,
TechnoBabble28