As if it wasn't made clear during the live SmackDown broadcast, Scott D'Amore's tweet pretty much confirms that there is some sort of relationship between WWE and Impact Wrestling.
During SmackDown, Mickie James was announced for the 30-women WWE Royal Rumble match and she was specifically referred to as the current Impact Knockouts Champion by Pac McAfee. Impact was also referenced in a tweet from WWE's official social media accounts.
D'Amore quote tweeted WWE's tweet:
"Well, well, well… congrats to @IMPACTWRESTLING Knockouts WORLD Champion @MickieJames and congrats to @WWE for finally walking through the forbidden door. Welcome to the party! See you at #HardToKill tomorrow on @FiteTV"
It's unclear if there's more planned for this partnership or if this is just a one and done situation. WWE needed to bring in outside talent because they've released so many people in the past year that there's only a handful of women on the main roster. There are some good wrestlers in NXT but most of them are not ready.
This is not the first time that WWE and Impact have worked together. When Ric Flair worked for Impact/TNA, WWE worked out a deal with them so they could get Flair inducted into the Hall Of Fame. In exchange, Impact got to book Christian Cage for a pay-per-view.
WWE also worked out a deal with Impact a few years ago for rights to use some of their footage for documentaries. In exchange, WWE referenced Impact on their WWE Network programming.