regex - modifying an exists file with install.ps1 via nuget-package installing -

i want edit startup.cs file when nuget package installs package. know need use int.ps1 or install.ps1 files don't know correct functions , properties own purpose.

i using c# .net core 1.1 , vs2017 enterprise

my questions

1. if doesn't exist, want add using $namespace$.my line in startup.cs file. possible?

2. also, if doesn't exists, want add codes in configureservices() method exists in startup.cs file.

3. is there way call init.ps1 , install.ps1 testing without installing package?

4. if need write own script, there used regex example nuget package this?

thanks.