HTMLDocumentItem hdi = new HTMLDocumentItem(ActionDocument, "<P@List name=\"YourField\" @P>");

String sChoiceArray[] = new String[]{"s1", "s2", "s3"};
ActionDocument.setItemChoices("YourField", sChoiceArray);

StringBuffer sHTML = hdi.getHTML(false);
//sHTML is the html required to make the combo box, this can then be put in a computed field on a page.

//you can also change the name of the field dynamically:
hdi.setName("NewFieldName");

Alternatively you can also use a temporary document rather than ActionDocument, eg:
HTMLDocument docTemp = new HTMLDocument(pSystem, pSession);