Is it alright to list ECs that were not on your primary application on secondaries? In the month I was waiting for verification, I took up a pretty major job and I'd like to mention it.
Yes, absolutely. The "any additional information" section is ideal for this. If there is no such section, you can use the school's updates system to incorporate it. Some schools do not accept updates pre-interview though, the ones that come to mind are USC-Keck, Mayo, and CMS @ Rosalind Franklin.
If you don't tell them about it, they won't know you're doing it!