Hey Paul,

Modo can natively open .SLDASM and .SLDPRT files.

The Plug-in that Dkron mentions, is a Sub-D to NURBS exporter.

The one that I was thinking of was Power Translators
http://www.thefoundry.co.uk/products/modo/plugins/power-translators/
You can choose to tessellate the solids or leave them as is.
You can import other formats as well.
It's a very well rounded CAD neutral importer(beta team), though I don't use it myself.

I have used DataKit for a few one off conversions and it didn't cost a dime.
Things may have changed since then. That was10 years ago.
I would assume that if you are going to be doing a lot of conversions, then you might want to look it.

SimLab Composer has a fair amount of CAD import formats, including .SLDASM
But as Danny said, the are no CAD neutral export formats.

What I would suggest is that you have the file sent to you in either STEP or IGES
I have found that MOI will open a IGES files that no other converter would.
I have used Deep Exploration, PolyTrans, Anark's Core Workstation, DataKit and a few others.

Be careful about the file size of the IGES file, as MOI is currently on a 32 bit application.
You may need to have the file broken up into a few parts.

That's my two cents worth.

Scott