Your data is your data. :)
There are a few ways to access the data.
1) You have the data in the file system in the SQL format.
2) You can export the model with or without graphics in the SQL format.
3) There is a java API such that you can access the data in memory using java plugins (eclipse extensions).
4) Buy/build a model compiler.
The format for 1 and 2 is open and published. The xtUML Editor delivers the xtUML meta-model in source model format. If you go to Help->Welcome->QuickStart you can create an example project with the xtUML Metamodel.
3 requires that you sling some java.
4 requires a license to a model compiler.
I hope this helps.