I do not know WS18900052 but I will try to help:
1. usually you can make changes to standard templates using transaction swdd_config, however changes make in this transaction are limited (you can't make changes to the template structure), any other changes will require copying the template and creating your own (think of what will happen in an upgrade - changes will be overwritten). most workflow templates use starting events, if this is the case you can change the linkage in transaction swe2, if not you will have to look in the process customizing or BADI's.
2. workflows are object oriented - WS18900052 is probably using an object or a class, you can use the data which already exists in them or enhance them (create a z subobject and delegate back) to get your data by adding attributes and methods.
3. that is a question for your HCM consultants.