Greetings Sonavi,
What you are articulating is the "otherside" of the BA dilemma that we all face from time-to-time. Most of us BAs are good at the Business <--> BA interaction; but we do not have the requisite skills to address the BA <---> IT interactions. Other BAs seem to be better at the BA<--->IT interactions and again struggle with the Business<--->BA interactions.
What you are describing in your question above is essentially the MODEL, VIEW, CONTROLLER architecture. VIEW = UI Interface; MODEL = Database and CONTROLLER = "business application logic".
There are many books that show how to get from usecases to a MVC architecture. A quick reference is Dough Rosenberg's ICONIX process (see http://www.iconixsw.com/EAICONIXProcess.html). In this book (website) Dough shows how to go from usecase <--> robustness analysis <--> sequence diagrams etc. You need to know some UML - some knowledge of component and deployment diagrams may be needed. A "DFD Diagram just wont cut it!
When writing user requirements this question normally pops up in the non-functional requirement section under constraints. eg. "The new system must conform to our adopted 3-tiered architecture"
May I suggest that you partner with an IT person, much in the same way you partnered with the business person to capture requirements, and start doing some high-level MVC (UML - diagrams)
All the best!
warm regards,
K