Sure.. external_ids are stored in ir_model_data table

external ids are in form : module_name.external_id
so to get all exported ids for product you an run:
select module, name from ir_module_data where module='product';

Acourdingly, you can prepare and modify those records with sql update statement.. 

hope it helps...

No, not by default at least. It is also a bad idea, because OpenERP uses this idea in all sort of refences (in the many2one fields for example). Since the number is generated for you it is generally a bad practice to want to change it and/or base any business logic around it, other than using the api/orm.

For comparing products between 2 databases it is ofter better to compare another column, such as a required/readonly field.