This works fine:
os.path.join(dir_name, base_filename + "." + filename_suffix)
Keep in mind that os.path.join() exists only because different operating systems use different path separator characters. It smooths over that difference so cross-platform code doesn't have to be cluttered with special cases for each OS. There is no need to do this for file name "extensions" (see footnote) because they are always connected to the rest of the name with a dot character, on every OS.
If using a function anyway makes you feel better (and you like needlessly complicating your code), you can do this:
os.path.join(dir_name, '.'.join((base_filename, filename_suffix)))
If you prefer to keep your code clean, simply include the dot in the suffix:
suffix = '.pdf'
os.path.join(dir_name, base_filename + suffix)
That approach also happens to be compatible with the suffix conventions in pathlib, which was introduced in python 3.4 after this question was asked. New code that doesn't require backward compatibility can do this:
suffix = '.pdf'
pathlib.PurePath(dir_name, base_filename + suffix)
You might prefer the shorter Path instead of PurePath if you're only handling paths for the local OS.
Warning: Do not use pathlib's with_suffix() for this purpose. That method will corrupt base_filename if it ever contains a dot.
Footnote: Outside of Micorsoft operating systems, there is no such thing as a file name "extension". Its presence on Windows comes from MS-DOS and FAT, which borrowed it from CP/M, which has been dead for decades. That dot-plus-three-letters that many of us are accustomed to seeing is just part of the file name on every other modern OS, where it has no built-in meaning.