I know this is a Technical Support thread but I have to say that if you do find a way to decode a SHA1 hash then please do let me know as I would be exceptionally rich! 
All joking aside any system that wants to store users passwords in clear text I would totally 100% stay clear of. If you are dealing with a customer base then you really should be doing everything you can to provide them with a secure environment for their data. Not doing that could open you to some pretty serious charges in certain countries.
That aside as well and the fact that I haven’t used aMember (I know other users here have done so though) I’m just wondering at what stage aMember wants to have a clear text password?
Is this because you already have your site setup and users are already registered and now you want to implement aMember on the site?
If so then I would have thought that there might be a way to get the passwords over to aMember without trying to take years decoding them by maybe simply getting the user to update their password to something new? Don’t know if that is at all a possibility?
I’m just a little astonished that aMember would want the details sent in the clear like that, just really wondering at what stage it needs that. Might help to get users here who already have implemented aMember to understand what is going on and help out on this one.
I remember hearing once though, and this is going back many many years, that it would take a room full of the fastest Cray XMP computers around 700 years or something to even decode one hash so I really think that you might be out of luck on this one if that is what you need to do. 
Sorry that wasn’t really of any true help but just wanted to see why aMember needs them sent in the clear like that as I was thinking of using it on a site in the near future but if that is the case then I will definitely be steering clear of it.
Hope you get it all sorted soon though.
Best wishes,
Mark