If I understand your problem correctly then there are two possible ways to solve your situation.
It depends on the version of CloverETL Designer you are using.
1. Version 4.1.0-M1: there is a new component called ‘CrossJoin’ in this version. You can use it to join your single string from HTTPConnector with multiple strings from XMLWriter as it does not require any join key and simply joins everything from every input together. So the output would be each of your multiple strings with single string from HTTPConnector joined.
2. Older versions: in this case you will have to use Reformat where you add some kind of identificator (key) to your strings (from both sources) and then use ExtMergeJoin to join them together. The result will be the same as in the first case.