Adding external Ids to Partners in OpenERP withouth a new module
Add an integer field in res partner table for storing external id on both database. When data is retrived from the external server and adding to your openerp database, store the external id in the record of res partner in local server and also save the id of the newly created partner record in the external server's partner record. So next time when the external partner record is updated, we can search the external id in our local server and update that record. 
Please check the openerp module base_synchronization and read the codes, which will be helpful for you.
My Question is a bit complex and iam new to OpenERP.
I have an external database and an OpenERP. the external one isn't PostgreSQL. 
MY job is that I need to synchronize the partners in the two databases. 
External one being the more important. This means that if the external one's data change so does the OpenERp's, but if OpenERP's data changes nothing changes onthe external one.

I can access to the external database, and using XML RCP I have acces
to OpenERP's as well. 
I can import data from the external database simply with XML RCP but 
the problem is the sync.
I can't just INSERT the modified partner and delete the old one
because i have no way to identify the old one.
I need to UPDATE it. But then i need an id that says which is which. 
and external ID.
To my knowledge OpenERP can handle external IDs.

How does this work? and how can i add an external ID to my res.partner using this?
I was told that I cant create a new module for this alone I need to use the internal ID works.