OMG has a file format for exchange of UML models. It would be very helpful if this could import another programs XMI export (the graphic layout will not be very nice, unless you have someone really good at deciphering various tools non-standard extensions) but the model itself will be intact. I would suggest not even bothering with the graphics, just import the model and let the user deal with layout of the contents.
I would suggest focusing on the output of the Eclipse projects UML2 tooling, as well as that from the NetBeans platform. Sparx Enterprise Archetect has some funky things in their use of XMI, but that is another popular tool.
The Linux/Mono version seems to run well (openSUSE 11.3/KDE 4.6), and having a cross platform tool is a huge help.
Thanks
Thanks much for your suggestion! But unfortunately, I won't be able to work on NClass in the future, so I won't be able to implement any new features.