In reality its not really a whopper of an issue ... but more of a not always easy ... programming task.
Its been done in the past and will be done again - by many including myself.
The problem with most 'Existing Apps' scenarios is that the existing app is often not really designed is such a way as to be compatible with the DNN way of doing things.
A lot of the real issues some down to how the original app that you are looking to port was originally coded.
For example - does the existing app - have a consistent business rules and database access layer.
That is - does ALL the real work of the app happen in a well organised business rules layer - or are the rules and UI elements all interwoven.
Another example would be the Navigation system.
In your exist app is the navigation system data driven or hard coded - the DNN navigation system is closely tied to the creation of page / tabs of content - an as such - while all the DNN data is stored in a database - it is not a dynamically generated by actions the users make - menu / naviation system - but instead a fixed - user layed out and formated tree based menu.
Having said this - it is theoretically possible to create your own MENU provider in DNN to extend the menu system to more complex navigation needs.
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The thing to understand again - about DNN is that it is a FRAMEWORK - with a reasonably tight set of constraints on how it operates.
So when it comes to porting an app to DNN - the question that has to be asked is - WILL my app work the way I want it - if it is recoded using the DNN Module and Menu framework.
In some cases - the answer will be no or not easily - in others it may be yes with some work - in which case it then comes down to the NEED or WILL to make such a port.
For starters though as to how to go about porting - I would first suggest spending some time getting to understand how DNN works - write some simple modules that perform simple tasks. Get an understanding of how modules interact with DNN and how to use DAL+ or DAL based constructs.
There are a number of people around that are happy to offer guidance - BUT - really - lobbing into a forum and going HOW do I do it - without understanding what the system you want to port to is really all about - is a pretty good way to get off to a bad start.
Get into the DNN source code - its all there for you to explore - and try to understand how it all hangs together - once you do that you will be in a much better position to understand what the steps of porting your existing APP to DNN may include.
Also - have a look at the Professional DNN 4 book or the DNN 4 Module Developers Guide - available in the downloads section.
DNN is a very powerful framework that tries to be a lot of things to a lot of people - and for the most part it does that very well - BUT with such power - comes a fair deal of complexity - and for a Coder that complexity can be both a blessing and a nightmare.
Coding a simple module can be the mater of a couple of clicks but a complex suite of modules with tight DNN integration can be the work of months or even years.
Westa