The CoverPage parameter value should be the name of the page that you want to be the parent of the ChildCase. If the Primary page is named "pyWorkPage", and the Primary page is what you want to be the parent, you could hard-code "pyWorkPage". Best practice however is to use "Primary".
Simpler the better. Should not have to write a custom Activity do something basic such as make an existing case a subcase of another case.
Assume if the case being moved was already a subcase, its existing parent/cover case would remove the subcase's pzInsKey from its pxCoveredInsKeys ValueList, decrement its pxCoveredCount, plus be Obj-Save'd.