Yes , you can send the intent to any app you like but its upto the receiving application to handle it.
Few apps may crash receiving it.
The way
Make an intent
Intent i=new Intent(yourContext,Activity_to_which_you_to_send.class);
Put some data-if you want to
i.putExtraString("key","value");
or put using a bundle
Bundle b=new Bundle();
b.putString(key,boolean_value);
b.putBoolean(key,boolean_value)
Starting the activity
startActivity(i);
Set the package of the app
i.setPackage("com.whatsapp");
Example
if you want to find out the main Activity of an app
go to command line
type adb shell pm -lf
pick any one and try it by passing as a second argument to the intent constructor defined above and then call startActivity method.
hope it helps you.