Not working as I expect.
1) Created a simple document in Indesign with 3 coloured rectangles (1 spot, 1 RGB, 1 CMYK) plus some text in black.
2) Exported as PDF X-3.
3) Openend PDF in Publisher and created document palette from document.
Result: Having now 5 colours in the document palette. One seems to be white, but if I use the CMYK slider it has 1% Cyan (rest 0). When you click the spot colour rectangle and look at the colour it says spot colour. If you look at the colour in the palette the colour is not marked as spot. Unfortunately I can't convert this colour into spot again. Unless you are not going to re-assign the colours from the palette the correct colour information of the objects is still intact.
Other observation: Although imported the palette from the document the colours seem to have nothing in common. E.g. clicking on one rectangle I expect to see the colour marked in the palette, but this is not working. If you re-assign the colour from the palette for the objects the fill will be shown in the palette.
farben.pdf