!UPDATED! How do i set the "connect as" attribute in IIS 7.0 /7.5 (and specify user and password) once the webadmin module is loaded?

more information, when using get-item i see the values i want to change:
userName                   : USER@DOMAIN
password                   : THEpASSWORD
---------------------------------------------
PS IIS:\sites> get-item "sitename" | fl *

PSPath                     : WebAdministration::\\serverName\Sites\siteName
PSParentPath               : WebAdministration::\\serverName\Sites
PSChildName                : siteName
PSDrive                    : IIS
PSProvider                 : WebAdministration
PSIsContainer              : True
name                       : siteName
id                         : 3
serverAutoStart            : True
state                      : Stopped
bindings                   : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
limits                     : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
logFile                    : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
traceFailedRequestsLogging : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
applicationDefaults        : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
virtualDirectoryDefaults   : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
ftpServer                  : Microsoft.IIs.PowerShell.Framework.ConfigurationElement
Collection                 : {Microsoft.IIs.PowerShell.Framework.ConfigurationElement}
applicationPool            : DefaultAppPool
enabledProtocols           : http
physicalPath               : \\serverName\websitesproduction$\webSites\Redirection Folders\sitename
userName                   : USER@DOMAIN
password                   : THEpASSWORD
ItemXPath                  : /system.applicationHost/sites/site[@name='siteName' and @id='3']
Attributes                 : {name, id, serverAutoStart, state}
ChildElements              : {bindings, limits, logFile, traceFailedRequestsLogging...}
ElementTagName             : site
Methods                    : {Start, Stop}
Schema                     : Microsoft.IIs.PowerShell.Framework.ConfigurationElementSchema

thank you

Roy