Yes you can
First of all, the harpooning already accompanies a free bite attack, so you would not even need to make a complete attack. But according to the rules of the game, a complete attack would give you a second bite attack (possibly against a lower CA of the poison of the first bite).
The question arises in two things. Whether or not you can change the order of your attacks, as well as whether or not you can roll the target between attacks.
For the first, although I can not find any source document that says it, I think you can change the order. The reason the source material does not need to say it is that it has never been specified either that there was an order at all. A creature simply has certain attacks, they must be listed in order in the short description of the creature in the books, but when they gain natural attacks, they have no intrinsic order.
For example, consider a creature created by applying a few models that each give natural attacks. The model does not specify where in the order the attacks arrive because there is no order. And in some cases, you can even change the order in which the templates are applied.
For the second, I found this:
Free shares consume very little time and effort. You can perform one or more free actions while performing another action normally. However, there are reasonable limits to what you can really do for free.
So yes, you can take a free action between your attacks to attract your target.