The recently retired 40-year-old Dallas Mavericks superstar and his Swedish wife, Jessica Olsson, are working on getting their green cards together ... which is the first step in a VERY long process to becoming a citizen.

"So once we accomplish that, then you have to be a green-card holder for, I think, over five years before you can even think about doing that [becoming a U.S. citizen]," Dirk told the Dallas Morning News.

"So we're going to do that and see how it goes. But obviously our [three] kids were born here and they all have U.S. passports and the wifey and I have been on a visa for the last few years. So hopefully that's going to be accomplished soon."

According to the Dept. of Homeland Security, Dirk's right ... he can apply for full citizenship 5 years to the day of obtaining his green card.

Dirk has previously said he won't give up his German citizenship -- the goal is to be a dual-citizen.

"But we're living here [in Dallas]," Nowitzki told DMN ... "This is where life is. We're going to continue to travel a lot in the summers, but once the school year starts, my oldest [daughter Malaika] is going into first grade next year."

Dirk and his wife won't be able to vote or hold a U.S. passport until they become official citizens in 5 years -- but they're not worried.

Until then, Nowitzki will fill his days eating ice cream and chilling with his family.