I would do it similarly. Maybe only used not a powershell command, but a command line such as
copy /b a.txt + b.txt a.txt
or
type b.txt >> a.txt
PS. We have the option to append to a CSV file somewhere on our product roadmap either for v3.6 or 3.7. It would save extra Run Program transformation.
Thank you, using copy or type is good. I had initially experimented with Powershell since I was going to try and ignore the header if the file existed, but realized it’s easier to just filter it out in EasyMorph after the fact.