imdb’s data comes from its readers, so it wouldn’t surprise me if someone was having a little fun and submitted a change to the Dallas page to make Grace’s last name “Van Owen”.
I've checked various scenes in which Grace was introduced to new Dallas characters (new to her, of course) but the most elaborate description I got was Assistant Grace. Later, she told Jack that she was an engineer. He already knew her as "someone from Marinos Shipping" and naturally that had to be Princess Daisy feeding bread to little duckies. Must have been some kind of code.
I haven't watched everything so I don't know if her credit changed to "as Grace van Owen" in later episodes.
It's possible that an IMDB user changed the name just for the fun of it, or maybe he/she was subconsciously inspired by the "van" in "van Kamp".
Pam becomes a Ewing Oil tycoonette and the first thing she asks Phyllis to do is to make her a cup of herbal tea. What a crap tv show!