you are thinking about it wrong...you should create a new role that has db_datareader writes, and then any additional rights you want toa dd. don't try to change the existing system roles.
Also, I belive every login has read rights to master, and read/write to temp by default. Are you sure that's what the issue is? maybe your login doesn't have execute rights to teh stored procedure that would be creating these temp tables. We might need an example of what you are trying to do.
Lowell
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