i've never had a problem with Blender interpreting existing UV maps.  you can select loops by holding down alt when you click (in case you didn't already know), and you can select edges in your UV map window.  i'm not yet sure how, if it's possible at all, to go from those edges to marking seams.  i'm not sure you can port seams without UV maps from one app to another. and if you can, i'm not sure you can with Blender.