Can this be solved by using custom membership provider?

hi

asp.net 3.5

I'm developing a website. The users are registereted inside another datasystem, which I only have access to via a xml file...
When the user tryes to login the system makes an http request to a URL and get information about the user....

So how can I create and use a custom Me,bership provider in this scenario. where most users exists inside another datasystem, not all but some exists in the local database.. There is a few users registered in the local database using SqlMembershipProvider (sqlserver2005 database)....
Also it have to support roles, but I cannot makes changes to the other datasystem... I thought of when user logs in then create the user in the current database... But that is a bad because for each time the user logs in I have to check the other system if the user has access rights...

How can this be solved using custom Membership Provider?
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