The connection parameters table - dbo.ConnParameters - lets you define different accounts to use for certain collections. For example, if you want to use a different account to connect to Server2 for the SysObjects collection, insert a row to dbo.ConnParameters: 

INSERT INTO dbo.ConnParameters
        ( InstanceID
        , CollectionName
        , UserName
        , PWord
        , AppName
        , IsActive
        , Comment
        )
VALUES  ( 2  
        , 'Collector.SysObjects'  
        , 'Server2User'  
        , 'specialpassword'  
        , 'Minion: Collector.SysObjects'  
        , 1  
        , 'Server2 needs to use Server2User'  
        );

Note that UserName must be a SQL login. And, this only applies to certain collections, including: 

• Collector.SysObjects
• Collector.SysColumns
• Collector.IndexStats
• Collector.Logins
• Collector.DBUsers
• Collector.WaitStats
• Collector.ServiceProperties
• Collector.ErrorLog
• Collector.DBFileProperties
• Collector.DBProperties
• dbo.ReplPublisher
• Collector.MBImportAllLogs
• dbo.MBSettingsImport
• dbo.MRSettingsImport
• Collector.MRImportAllLogs
• Collector.AGProperties
• MRImport
• MailInfo
• JobInfo
• MCImport
• MBImport
• Collector.TableProperties
• Collector.LogonAudit