If it supports StdIn for input, you can try piping the path to it (e.g., “path” > executable.exe). Otherwise, PowerShell wouldn’t really have a way to respond. Keep in mind that, when your external program is executing, it’s doing so under Cmd.exe, not under PowerShell. You’re “out” of PowerShell at that time.
Now, that said, a Process object does have a StandardInput property (see http://msdn.microsoft.com/en-us/library/vstudio/system.diagnostics.process). Possibly you could use that – again, IF the application accepts input via StdIn.