Usually people use .plt for isodraft underlay, here are three methods for generating plt.
1. Pdms Command ( PLOT = D:\AVEVA\Plant\PDMS12.0.SP*\Plot\Plot.exe, Video) :
SYSCOM ‘PLOT PDMS C:\plot.dxf C:\plot.plt‘
2. Another good method is to use E3D, it can import dxf files without any difficulties and generate Draft Library.
3. Third method is to use Autocad and Autolisp script supplied with PDMS