Look into sp_MSForEachDB (hidden MS procedure)
sp_MSforeachdb 'if ''?'' NOT IN (''tempdb'',''master'',''model'',''msdb'') use ? EXEC sp_droprolemember N'db_datareader', N'Jack'
sp_MSforeachdb 'if ''?'' NOT IN (''tempdb'',''master'',''model'',''msdb'') use ? EXEC sp_addrolemember N'db_datawriter', N'Jack'
Otherwise you can always look through a cursor of all User Databases