Sorry Julie but Captivate only really edits files created in Captivate.
The only option I can think of is that your users might be able to use a screen capture tool such as SnagIT to take screen captures slide by slide and then send the resulting set of PNG files to you for you add to slides in a Captivate project and thereby assemble a demo. However, the downside of this approach is that it's VERY labour intensive and your remote users would need to be very careful that all of the captures were the correct size and format, and not miss out any steps. Otherwise it would never work.
If they are only ever going to need to do this once, I would suggest they download the Captivate trial version and use that to do a capture project to send to you. But since you have only Captivate 8 and the only downloadable trial version of Captivate now is version 9 that's not going to work for you either.
If this situation is going to come up a lot, my gut feeling is that your end users might simply need Captivate as well.
It's not a short term fix, but I went through this working for a global automaker.
We did a bit of cost analysis to justify a license for the subject matter expert, or SME. The users wouldn't be recording their work, just a select few would work as SMEs.
When we considered the travel time, cost of a laptop, and loss of my productivity to sit with a SME and record their interaction with a web application, it more than paid for a license of captivate on the SMEs workstation.
Since you are working with multiple SME's, you might be able to purchase a license and a laptop to send to their location and have them record the segments needed for your edits. Keep in mind that you will need to build a job aid (documentation) for a SME to launch Captivate, start the recording mode, do their work, end recording mode, and save the file. It's best to work with someone in your building to get this started, test their work with you in the room, work through edits of the Cp file they generate, apply final edits to the job aid and only then send the tools to a remote user for self-recording.
This is an issue we constantly grabble with as well.
I have often thought that this would be such a great thing for Adobe to create - a simple app that allows SME's to record screens and save them in a format the training developer can open them in Captivate. It is sort of like the reviewer tool they created. In my opinion it should be free like the reviewer tool. No editing would be allowed and no publishing so the screens captured could only be useful in conjunction with a licensed copy of Captivate.
Is there a way to request this to the development team? It would be a HUGE selling feature as more and more software training departments work this way.
There's a form page on the Adobe website for enhancement requests and bug reports.
Knock yourself out!