Yes, you should be able to add it to your current SSDT project. The person that sent you that package file may have deleted the connection manager from the package before sending you the file as it may have contained username/password information.

You might try creating a new package and adding a connection manager to it then compare that to the other package and see if any sections of XML are missing that deal with the connection manager. You might be able to put in some "dummy" information so SSDT will open the package up.