I very much doubt they are aware of FreeCAD. Even though it enjoys some popularity (when the project was hosted on Sourceforge, the downloads exceeded one million - now on Github there is no easy way to get stats) it remains pretty much unknown in the industry. Open source software in general is not well-known.
I'd tend to agree with Sinclair, candidates who know the CAD system used by the engineering company may have a head start. But it always depends on how convincing you are at the job interview. Also If you list more than one CAD system in your resume, they should infer that you have a capacity for learning.
Honestly, this depends on your adaptability. Some people struggle to learn new things. Some beginners come here with no prior CAD experience and learn FreeCAD very quickly. Others with long CAD experience struggle with it. And there are all the others in between...