FPS Creator is not an add-on, it's a standalone product. If you have model packs for FPSC, you can load the models in Dark GDK (models only, not the whole game that you create in FPSC, you'll need to re-program that). Some people on the forum might have figured out how to load complete FPSC levels in Dark GDK but I haven't seen such topics in a long time and I don't know if it's still possible.
Dark GDK .NET is again a different product (and a different programming language). The purchase page clearly explains the difference between the licensing options: Dark GDK is free but you need to buy a commercial license for 30 dollars if you want to release commercial games made with it. Dark GDK .NET costs 30 dollars by default but it already contains a commercial license.
http://www.thegamecreators.com/?m=view_product&id=2128&page=upgrade
Some plugins that are available for Dark Basic Professional have Dark GDK versions but not all of them. You'll need to decide which plugins you will need and find out one by one if they have a DGK version or not. (There is a page on the TGC web site which lists plugins for Dark GDK but it may not be completely up-to-date, it's better to check the pages of the individual plugins to find out which versions they have.)
If you buy a plugin that has both versions, you probably get to download both versions. For example I bought Dark Lights and I was offered both the DB Pro and the GDK version for download. I have no idea which versions are in Bonanza.
About Dark Voices: I once briefly tried its demo. The part that generates the phoneme files is a standalone program, not a plugin, so that will work, but the codes examples they give with it are implemented for DB Pro. Maybe it would be possible to program an importer which reads the phoneme file generated by it (after all the feature page says "Mouth data stored in a text file, easy to parse by many languages"), I haven't looked at it that deeply. If they give code examples for DB Pro, it should be possible to convert that for Dark GDK. However, to use Dark Voices in a game you also need character models which are prepared to move their mouths according to the phonemes, otherwise it's no use.
EDIT: Just edited out the reference to PureGDK as it was off-topic.