The Tridion oData web service is used to query content, not to insert content in the CMS. If I understand correctly your question, you would like to be able to retrieve the Keyword Metadata by querying the oData web service.
This is possible by querying the CustomMetas oData entity with for example this C# Expression query in Linqpad http://www.linqpad.net/ :
from x in CustomMetas
where x.PublicationId==27 && x.ItemId==3326
In this example 3326 is the Keyword ID.
Corresponding RESTful URL query:
http://yoururl.yourdomain/odata.svc/CustomMetas()?$filter=PublicationId eq 27 and ItemId eq 3326
You need to Publish the Category containing your keywords to be able to find them via oData.
Here is my keyword in Tridion: