Transferring bookmarks from Apple Maps to Google Maps
There is no way to synchronize Maps with Maps (so to speak), but you can sync Earth with Maps, with Google Earth being on the iPhone. If you log in to Google Earth, it will synchronize your maps. Those can be added from Google Maps on the web. Unfortunately, there's still no way to add stuff from an iPhone, but there isn't no connectivity between them.

Because I think that was a really messy and hard-to-understand answer, here's a clearer synopsis:


In the Maps app on the iPhone, there's no way to see things you've added on a computer.
In the Google Earth app on the iPhone, you can see "maps" (collections of pins) that you've created in Google Maps on a computer.
There's no way to add pins on an iPhone in either of these apps to later see on a computer.


I'm sorry there's no way to do what you want in a native Google app!