No. It's too late for the tests to affect the design, so you can't get the major benefit of TDD.
What you can do, however, is to write tests as you work on the code. Say you've got a method that's too long. Write a test of a little piece of it, assuming that little piece had been extracted into a new method. Now extract that method. Or say a bug report comes in; this is an opportunity to improve your design (and incidentally your test coverage). Write a regression test that demonstrates the bug. Study your code to understand the root cause of the bug. Now write a good unit test that expresses how that root cause should be behaving. Make that test pass, and if you did it right, the regression test will be passing, too.
TDD tests really need to happen (a) in advance of; and (b) very close in time to the code that they are testing. So accept that it's too late to retrofit your code "for the sake of TDD"; just get into the habit. With the practices outlined above, the code that needs the tests will get them.