I think CurrentItem.To_ should be assigned a fully-resolved email address (or list of addresses), eg.
CurrentItem.To_ = "email@example.com"
CurrentItem.To_ = "'John F. Doe' <firstname.lastname@example.org>"
To do what you require (assuming your address book contains the entry “John F. Doe”), enter text directly into the To: field on the composing message view:
call GroupWise.ItemSetText("X00", itfTo, "John F. Doe", TRUE)
I hope this helps.