I have a Polish national type D visa, and would like to stay in Italy for more than 90 days in 180 day period. I was actually wondering if its possible considering there are not border checks betwen Schengen countries?
You shouldn't do this. The Polish Ministry of Foreign Affairs says:
A D-type national visa permits to enter the territory of the Republic of Poland and to stay continuously or for several successive times in this territory for a total duration of more than 90 days during the period of validity of the visa, but not longer than one year.
A D-type national visa also permits to travel in the territory of other Schengen Area Member States for up to 90 days during a 180-day period, during the validity period of the visa.
In short, you can stay in Poland for the duration of the visa. Elsewhere in the Schengen area the 90/180 rule applies.
The absence of border controls may mean it is possible, but member states can and do perform checks at borders and elsewhere on an ad hoc basis. Being caught breaching the terms of your visa will make life difficult, at best.