Is it possible to merge contacts from different sources and upload them to GMail?
This answer is based on comments from Niall C.

There are 5 phases in the process :


Select each contact, and then link them when relevant. The goal is to have 1 friend = 1 contact.
Export the list (Contact > Menu > More > Import Export > To SD Card), it will work even if you don't have any SD card.
Transfer the file to a computer (the file is quite light, even with GPG keys and user pics)
Open GMail, backup your contact by exporting them
Import the VCF file


And that's all ! Your contacts in GMail have now phone numbers ! Don't worry, GMail merge contacts becase their email adress is already known.

Source : How to move phone contacts to Google?