Which software to design alternative casings

[Pander]

Hi all,

Soon I'm granted some serious playtime on a 3D plotter and I would like
to create alternative casings for FreeRunner. Now, I am looking for open
source (Linux) software to design my alternative casing.

I have downloaded the CAD files and they comprise of:
- Pro/Engineer Wildfire 3.0 files .asm and .prt
- IGES (Initial Graphics Exchange Specification) files .igs
- STEP files .stp

Can anyone tell me which open source software I can use to work with
these files?

I have discovered that BRL-CAD can import ans export IGES, an .deb
package is already on its way:
https://bugs.launchpad.net/ubuntu/+bug/311405 However, I have no idea
how to get started on this ;) Any suggestions are welcome.

Regards,

Pander