No it’s not possible to be migrating the files from a SQL 2000 to SQL 2005 instance with the files still attached to the SQL 2000 server. A SQL 2000 to SQL 2005 migration should go very quickly. Only the meta data within the database is being changed. The physical data isn’t being modified at all.
Take a backup of the database and restore it to a SQL 2000 server, then detach it and attach it to a SQL 2005 instance. The amount of time that it takes to attach is how long it will take to convert the production database from SQL 2000 to SQL 2005.