External Identifier support for field matching on imports

Users need to be able to import Contributions, Activity History records and Membership Records using an external contact identifier as the "match to contact" criteria.

Implementation
============
1. Add unique index on civicrm_contact.external_id in the schema (add to 1.6->1.7 upgrade as well)
2. Modify import mapping form and post process for the 3 import types above to support "match to contact" for the External Identifier field. (This is in addition to the current options to "match to contact" via First Name, Last Name, Email, Contact ID)

NOTE: This feature is NOT supported for importing contact records, only for these 3 types of "auxiliary" records (although a user could configure their dupe matching rules to use External Identifier if desired).

— original post from Tobias —
As per the lobo post on civicrm-dev (pasted below) please make it possible to match the "external identifier" field when importing contacts data or activities. Kabissa will be using this field extensively to import organizations, individuals and activities as we migrate from our current modernbill setup.

Thanks!

Tobias

On 11/29/06, Tobias Eigen <tobiaseigen@gmail.com> wrote:
> Hey folks,
>
> Was glad to see the activity import feature, which I want to use to
> import the event_log table from modernbill on Kabissa's current site.
> This table simply has five fields:
>
> log_id
> client_id
> log_type
> log_comments
> log_stamp
>
> I face two problems:
>
> 1) I'd like to match the client_id to the "external identifier" for
> the organization. That field is not available to match with.
>

can u file an issue for this in 1.7. Seems like we might need to
elevate external identifier to "contact id" status to synchronize with
use cases like yours.

> 2) I have the funky unlx date issue once again. log_stamp is in the
> form 1050163720 (this is equivalent to 2003/04/12: 11:04:40)
>

follow henry's suggestion and do this in excel