[aram]

Yes, you can create additional users within Piwik and grant them access to specific websites.

[PavlovsCat]

Looks like it's possible to do this programmatically even:

http://piwik.org/docs/analytics-api/reference/#UsersManager

http://piwik.org/docs/analytics-api/reference/#SitesManager

(and if there are some quirks in the way, file a ticket etc.)