As far as I can tell there is not document on teaching how to use Powerdesigner API.
Engineering provides some examples in online document. Excel Import Tool you are using is written in vbscript. It contains a lot of information on how to call an API. Some extensions also contain script. Finally Metamodel Objects Help (In PD go to Help) is very useful.
It'll take a while for you get used to the API.
Here is my latest example.
Create a reference between two tables (parent and child). Also using existing column in
child Table(childAddress) as foreign key.
set model = ActiveModel
set references = Model.references
set parent= model.findChildByName("parent", cls_Table)
set Pcolumn= parent.findChildByName("parentAddress", cls_Column)
set child = model.findChildByName("child", cls_Table)
set Ccolumn= child.findChildByName("childAddress", cls_Column)
set newRef = references.CreateNew()
newRef.name ="ABC"
newRef.code ="ABC"
set newref.childTable=child
set newref.ParentTable=parent
set Joins=newRef.Joins
for each j in Joins
set j.ChildTableColumn=cColumn
next
ActiveDiagram.AttachAllObjects