Dm add7?

What's a Dm add7 chord? Even in this site isn't listed!

I think that's the same as Dmmaj7, so a minor major 7th chord with steps 1-b3-5-7,

That is because it is incorrectly named. Like paerdeveygh said, it could be a Minor Major 7th chord(1,b3,5,7), or it could be a Minor 7th chord(1,b3,5,b7).

Yeah because doesn't 'add' in a chord mean that you take the normal chord and then simply 'add' on the stated extension, while missing out the ones between, i.e. add 9 = 1,3,5,9 (miss out 7th). So the site is technically right because if you add7 there is nothing to miss out between the 5th and the extension so in this case it could be either of those that Moonlit said? Have I got that right guys? Guys?

Yes. Often you are supposed to have it as the highest voicing too, although that is not always possible on a guitar. Also, adding a 7th onto a chord with a root and third in it makes it some type of seventh chord. Therefore, "add7" doesn't exist.