In AutoPLANT modeler, most of the companies use their standard template for generating Isometric. OPIM uses default seed file (isoseed.dgn) as template for generating isometric. Any file with extension excluding .DGN will not support it.
Following steps will aid user for converting existing dwg template to default seed file.
C:\ProgramData\Bentley\Plant V8i\Bentley Plant V8i\Projects\SAMPLE_METRIC\Config\OpenPlant\Isometrics\styles\IFC\config
http://communities.bentley.com/products/plant/design___engineering/w/plant_design_and_engineering__wiki/12636.setting-file-type-dgn-or-dwg-and-paper-setting
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