Usually if I have an .app I want to import into Munki, I just use munkiimport nameofapp.app, and Munki automatically creates a read-only disk image (.dmg) with the .app inside. For some reason, with the mBlock.app, I kept getting a 5341 error. I tried this workaround, but I still got the same error.
What worked for me (your mileage may vary, of course) was just using Disk Utility to create a read-write disk image (of about 500 MB, becuase the mBlock program is a little less than that), and then converting it to a read-only disk image.
I don’t believe it has to do strictly with the large size of the app, because I’ve used Munki to import Xcode, which is typically 5-6 GB, much larger than a roughly 490-MB app.