I had the same problem (see http://communities.mentor.com/message/11879#11879) and did not find any solution up to now. When you export your design, it should be possible to define the units (mm, inch, th), that are used.
you can ascii out without decals. When you read in the ascii then PADS is missing the decals in the library. Then PADS is looking into the library and loading it from here.
Since V9.2 you also can use the builtin "update from Library" in Tools.