Hello,
If you need to add new rows from JavaScript, look here:
http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=7700
I recommend you to have a look in three places:
a) Infragistics knowledge base
http://devcenter.infragistics.com/Support/KnowledgeBaseResults.aspx?type=Full&query=webcombo&articletypes=0&age=0&sort=LastModifiedDate&samplesonly=0
b) The old Infragistics forum
http://forums.archive.infragistics.com/?heirarchy=infragistics.products.netadvantage.aspnet
c) Have a look in the help documentation file installed on your computer. Search the JavaScript functions that add new rows to an UltraWebGrid.
This is because the webCombo uses an UltraWebGrid for its dropdown portion.