I have not tested this but it seems to be storage profiles are your solutions.
When you import something you should be able to tell it what storage profile to use. By default "Any" is used - and this contains all your datastores. So it picks the one that appears the most empty.
I would think you would had a profile with only the datastore you want to use as an option it wouldhave no other place to put the VM, so it would be imported.
Sorry I have not tested this, but sounds like it would work.