Dear Ingmar,
as you know, gopenmol can convert cube to plt - but perhaps there is a way to use that program to do it the other way round... although I doubt it.
The next release of Turbomole will contain an option to print out the 3D grid files directly in cube format. And TmoleX will be able to read in plt files to plot orbitals, densities, vector fields, etc. by using OpenGL.
Regards,
Uwe