Short answer: You can't.
The longer, more correct answer: You can move everything from
/etc/profile and equivalent to
/etc/bashrc and equivalent (details depend on the shell(s) you're using), and have the former source the latter; that will get you as close as it's possible to get (with the caveat that some things may need extra if/elif/fi tests if they should or shouldn't be run for login shells).
The extra hyphen is a short-hand for "-l" or "--login", which tells
su(1) how to invoke the shell: By convention, a shell knows that it's a "login" shell if its
argv (the name it gets told it was started as, which does not need to be anything like the filename of its binary) starts with a dash; for example, bash will find itself "called"
-/bin/bash. You can confirm this by logging in on the console and running
ps o pid,comm,args $$ - the "args" will start with a dash. A login shell uses a different set of startup files, which is the major difference; see the man-page for your shell(s) for details (e.g. bash invocation). There are some remaining subtleties about the environment that's set before bash ever starts, but that can all be fixed in those startup files.
At this point, it's useful to take a step back and consider: What problem are you trying to solve? Changing your identity shouldn't be something you routinely do; if you find yourself needing it, you should try to figure out how to fix the underlying thing you're trying to do not to require it.