I think u are looking for a programm like DIA
http://freshmeat.net/projects/dia/
Dia is a program for creating diagrams of all kinds. The current version can do UML class diagrams, Entity-Relationship modeling, network diagrams, and much more. The engine is very flexible and dynamically loads diagram-types from disk. It can currently export to PostScript and load/save in an XML format
That should help ....