Hi Matt, you don't have to be in the Cat project to create categories. Migration ones are TRICKY, and I don't know if you've ever seen Steve Harris' handy dandy migration tool. It even lets you press a button to see the CIB you can add. All you have to do is copy it.
mcg.harrislineage.com/index2.html
Just enter the country and the proper admin entity in the boxes and click submit.
I entered Ireland, County Down and New Zealand, Otago and VOILA, I see the category should be [[Category:Migrants from County Down to Otago]] and then I click the CategoryInfoBox Migration button and it displays {{CategoryInfoBox Migration
|fromCountry= Ireland
|fromEntity= County Down
|toCountry= New Zealand
|toEntity= Otago
}}
for me to copy/paste into the new category page.