I think you can, because one method of filling grain prior to finishing is to sand Danish oil to create an oil slurry of the oil and sawdust, and push that into the grain. It would have to be cured beyond doubt though before applying anything else on top.
Poly is a difficult lacquer to work with as it takes a hell of a time to cure properly, as Ive been finding out myself on another thread!
Car lacquer as recommended to me on here is much simpler and cures a lot quicker, it doesn't quite create the lovely glass like finish poly does though