Due to everything being binary in the Mentor tools it is impossible to do any types of data migration, even if you got the ASCII import export from Mentor you would not be able to bring all data over. I would suggest taking the Gerber data from the board you want to get into Expedition. Then Expedition allows you to import Gerber data then you can trace out each layer using the Gerber. This is what we do now. The things to keep in mind are grids and datums, make sure to place on a good grid system so that when placing parts over the Gerber layer you don't have to reduce your grid so low that it is impossible to place the parts on top of the Gerber outline.
Hope this helps.
Use the PADS translators to import the Altium database. Then use the PADS to Expedition translator to generate the files to be imported into Expedition. Create a new Expedition database and import the files from the PADS to Expedition translation. After the importation, open the Padstack Editor and check the padstacks for soldermask and solderpaste pads. Add the pads to the stacks if you need them or do some work-a-round in CAM. Check the plane shapes, if when you fill then they do not fill as expected. Might have to re-declare some shapes as planes or the net name. Remember: If any of the ECAD systems does not handle some attribute or element or concept, then those will not translate through.
So if I import a gerber of the traces from one design into an existing expedition layout that has been annotated and has netlists, is it possible for Expedition to "adopt" the net name to the newly imported drawobject (gerber)?