You should be able to do it from within SlickEdit now. The color coding is configurable within the editor, and allows nesting of token regexes. See Tools -> Options -> Languages -> Database Languages -> PL/SQL -> Color Coding, and go to the Tokens tab and look at the Doc Comment entry for /*+ */ for an example of that sort of nesting.
So what you can do is:
1) Go to Tools -> Options -> Languages -> Language Manager and click on Add Language
2) You'll get a dialog box allowing you to give a mode name, and file extensions. You've got a couple of choices here. If you think PL/SQL is close enough to PL/pgSQL, you can choose to copy the settings from PL/SQL, and then you may have less work in editing existing color coding settings to match the Postgres language. This would also give you smart indent, although small differences in the languages may make it so it may not recognize some contexts where it should be indenting. On the other hand, if you want to start from a clean slate, then leave the "Copy settings from" checkbox unchecked.
Once it's created, you can add tokens to the color coding configuration, and when you open files with the extensions you associated with it, it should automatically go into your new language mode.