Can I import bpmn model from Aris Express into BPM Studio ?
Not familiar enough with Aris Express, but in the earlier 11g versions you could use a tutor extension in JDeveloper to import some different models.
Since PS5 or PS6 it's possible to import them without the extension and this can be done in the BPM Project Navigator view.
Right-click on your BPM application and you can select "import models", the feature supports Oracle Workflow, Visio, Tutor and XPDL which is hopefully a format you can use to export from Aris.
Hope it helps.