I'm not really qualified but this is my two cents:
If you declare TMyForm = class(TForm) end; in interface section, add {$R *.lfm} in implementation, and provide the namesake .lfm with object MyForm: TMyForm / end (on two lines) inside, I *think* the IDE will pick it up (did not try it).
But the way I'd go about it (if you really need to) is, backup your existing unit, create the new form in the IDE with the desired name, copy-paste code from the old unit to the new one.
Then again, I don't know what exactly you are doing but I guess you'd be better off simply using the existing 'code' unit in the new 'form' one, so that form-related code (mostly event handlers) is in one place and program logic (the existing unit) in another file.