Diluting or thinning uncured Milliput
You can use water to dilute it.

Here someone suggest adding a little soap with the water, as this will not only let it mix better, but also increase the drying time, although I think this might be detrimental for the structure.

Note however, that some users complain about the manipulation of this mix (e.g. here: "I tried [to] dilute Milliput and although it worked (sort of) it was messy and time consuming").
The ratio really depends on what you want to achieve - what works best for your application. I suggest experimenting with the amount, slowly adding more, until it behaves the way you want it to.




Alternatively, you can opt for using ProCreate Putty, which I saw mentioned here, and sounds like something that acts a bit more like you would expect of a watered down putty.



Additional sources:
http://www.40konline.com/index.php?topic=32579.0
https://privateerpressforums.com/showthread.php?76994-Milliput-knowledge-please
https://www.planetfigure.com/threads/thinnin-milliput.24159/
https://themostexcellentandawesomeforumever-wyrd.com/topic/79094-liquid-greenstuff/