If you're thinking of using it a bit like a wash, applied selectively, then yes you can.
I'd suggest something like:
1 part paint
1 part retarder
5 parts matte medium
5 parts glaze medium
5 parts water
The paint is your pigment (obviously), the retarder helps to avoid tide marks, the matte medium "thins" the paint opacity, the glaze medium is your flow release (and also helps avoid tide marks too), the water loosens up the whole mix so that you can apply it thinly.
It takes a while to dry fully and for the mediums to go fully clear (24 hours at least), but you can build up layers for colour effects and to increase opacity. Remember, it's best to go fairly thin as you can always add more layers - but if you over-apply a heavy colour in any one layer and then it dries, you can't remove it!
You can do it any stage, but I'd avoid doing it over gloss varnish so that the mixture doesn't pearl/streak. Afterwards, paint/varnish/whatever over it as normal.