Many thanks to you Jim! I tested your simple formula - and it works (ok, I only tested it with a few values, but each was correct).

I only had to add the width of the original map to the new isox...

Does anyone know a good function to convert normal 2d coordinates into isometric coordinates?

well, in my previous post cos30 and sin 30 are constants (~0.86 and 0.5) So it's pretty much the same (just a comment).