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Is there a way to convert a MSI to portable app format?
No there isn't, and for one main reason:
MSI files are installer files which basically act as a small database. Windows handles them by sending them to a program called msiexec.exe which reads the information included in the .msi and uses it to install a .exe file (and, usually, a bunch of other files) that is bundled within it.
PortableApps.com apps require a developer such as myself to manually run an app (that is already installed, when there is an installation process involved) and observe the changes it makes to the system and then set up the PortableApps Launcher to handle these changes.
Converting from a .msi directly to PortableApps format can't be done because (even if we were using an automated tool for the process) it would miss the important step of running the actual bundled .exe file to determine the changes it needs to handle from the program.