Can NOLOCK in a stored procedure be overridden when executing it?
No, query-level hints and options override session and server-level settings. Basically, a hint applied at the query level is the lowest common denominator, and wins (except in the case of MAXDOP and Resource Governor, but that's slightly different than what we're talking about here anyway).

This used to be documented for SQL Server 2000, but this verbiage has been removed from more modern versions of the documentation.