try selecting the drawing that has been imported and and then go to 'pens and colours' - i think that there is an option to show those only in use in the selected drawing . . .
8.1, (and with it plotmaker 3.1), has the ability now to override the archicad penset for particular drawings and stick with those you have defined. if you tend to adjust pens in plotmaker a lot, moving to 3.1 would be a worthwhile (and free!) upgrade.
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