With my wife being offered employment i assume that a temporary work visa will be required to begin the job asap?
Yes.
Will this be an extra cost or can we tag onto the existing application?
It will be an entirely separate application, for either an Essential Skills visa, or Work to Residence if the employer is accredited. (See the INZ website for details of those visas.) You can also apply for a partner-sponsored visa to allow you to accompany her
https://www.immigration.govt.nz/new-...s-visa-holders - you can put this in at the same time, linking it with a covering letter, and INZ will work it all as one case, supplying the visa number etc. which you won't know to begin with as it becomes available.
Are we also able to alter the principal applicant to my wife?
No. Although couples tend to think of an application under SMC as 'our' application, actually, it is the principal applicant's application, with the spouse as a (minor) etc. only.