Hello every body,
I'm working for a company, they have a project like this: upgrading the current web base system to a new one that have new function. The current system is a local site for a university at: www.hoasen.edu.vn (language is vietnamse, so I think that you may have problems when reading such information). That system costed 5 people, each with 2 months working full time. They have many modules:
User modules : Login, Search, Annoucement(with paging), forum , photo album, news, tracking student records  
Admin modules: Manage users, Manage forum, Edit/Add/Delete page, content, Manage news, Manage Photo Album.
The Forum module has many category and subcategory with large number of users about 5000. There are  6376 posts. The data is very large and password is encoded in MD5 format. 
Now the new system requires three modules: Small Document Management System, Agenda Management System, Event Management System.
I wonder if DotNetNuke can help me replacing the old one and build new modules that satisfy my needs or I have to code the three new modules?
Any idea will be highly appreciated. 
Thank you in advance. 

Well, I'm not quite sure, but it looks like you're asking if you should redevelop a website that was custom developed, to use DotNetNuke and DNN modules?

How much time do you have for this project? Would it be easier to just develop the 3 new modules for the old system? You'll be hard pressed to find the exact DNN modules you need for your specfic application, and convert everything over.

I'm not saying it can't be done, it can, but you need to be prepared to do a lot of customization, or pay someone to do that customization.

Let me know if you need any help.

Thank you very much for your idea!
After a hard working month, we have chosen the right way to do this project. We base on Exchange Server to use some functions and try to find down the api to deevelop new modules base on that portal.