Perfect Space Convolution Reverb

you have to clickon the icon that looks like a a little page with a corner folded over, usually means NEW in most programs. In this one that loads your convolution impulse. Dont feel stupid, I couldnt grok it until I worked my way through the SONAR power 5 book

No, the last versions including it were X3 Studio/Producer.

Anderton, do you know how to turn pwav impulse files into wavs?

I love the impulses that came with my Garritan Steinway by Garritan,
but they're pwav and I don't know how to load them into Cakewalk's REmatrix Solo convolution reverb.

I ask before in this forum, but no one replied...

I think the pwav is some form of protected/proprietary wave format the Aria audio engine uses for IR.  They aren't normal IR wav files(though they are RIFF formatted).  Looking at the ones that came with my Garritan ARIA Player I suspect they aren't all that complicated but I don't think there are any tools that will allow you to convert them (at least not available to the public).   Here's the engine developer site
 
http://ariaengine.com/
 
You might want to ask https://www.plogue.com/ if there is some way to convert/access the pwav files.  Otherwise you'd have to reverse engineer their format (which may not be that hard or may be totally impossible, hard to tell).  If you could find some way to apply the IR to a input sound (rather than only on a sample playback) you could resample the IR (I've done this with Melda Productions MConvolutionMB with other reverb plugins) but I doubt a resample will give you the same quality you are looking for.  I couldn't see anyway to apply the IRs to an audio signal with Aria player though. 
 
If you can find a sharp impact sample you might be able to use that as an IR source.  Find a very sharp drum hit and apply the IR you want, play a single note, trim the result wave file then convert that into an IR file using perhaps using https://www.voxengo.com/product/deconvolver/  Though again I doubt you'll get the quality you hear in the original samples.
 
There are lots of IR plugins and libraries, I suspect you can find something as good or better though (if you aren't looking for free).  I don't think there is any easy way to get at those Aria IR files.