ITNinja

You provide the certificate as option, as any addin.

Zip the MSI and the certificate file together and create a standard MI which runs the MSI, because it gets the MSU from local souce

If you have addin.exe or anything, which is not really an addin you can use scripting to install both one after the other or use 2 MI to install each of them separately and use a Task chain to run it one after the other (especially interesting if one of both is changing regulary)

And at last, the good old AppDeploy Repackager if all else fails.

Btw: it does not make sense simply adding "SPAM"-tags, since this may cause someone to oversee a question who is able to answer it.