Is it possible, for example, to create an image of an existing Mac running FileMaker 11 on El Capitan?
You can definitely create an image of your existing Mac. The question is whether or not you can then use that image to create a virtual machine in Vmware or Parallels (whenever I have looked into Virtual Box, it has always appeared to me to be a giant pile of time-sink and frustration, so I have avoided using it.)
Disk Utility can create an image of your hard drive. So can apps like SuperDuper or Carbon Copy Cloner. Instructions for doing so are easily found via Google, etc.
However, these images will be .dmg files, not .iso files.
The question is whether Parallels or VMware can be use a .dmg to create a new VM.
There are instructions online about changing DMG files to ISO files, and I am fairly sure that ISO files can be used by both VMware and Parallels.
So I would say that, at least theoretically, yes, you can create an image from your existing installation of El Capitan. But I am not convinced that the process would work in practice as well as it seems like it should in theory.
However, if all you are doing to use the VM for is to run FileMaker, so I'm not sure that it is either necessary or a good use of your time to try to do so.