prior to installing

Is it possible to call up my own program prior to installing and have NSIS take in values from my program?

Any given Commandline Value is stored in the variable CMDLINE. You can use the Function GetParameters to search for spaces. You can find the Function in Appendix B of the documentation

You can exectute an application using ExecWait. After executing your application you can get the return value or read files or registry keys which are created by the application.