Oh right, so you want to trigger the Interfits off the pop-up flash? Have you tried it?
The question is, and I don't know the D7000 well enough off hand, is can you switch the pop-up to full manual output? Just the main flash, no auto-TTL pre-flash? If so, the Interfts will sync off that via their optical slaves, but if there's a pre-flash, they will trigger off that first and fire before the shutter opens.
It's not a great idea though, as the pop-up's light will also influence the image. Better to get a cheap set of manual triggers, eg Yongnuo RF-602.