Can potassium nitrate and sulfuric acid be used to nitrate organics?
A simple, but dangerous path to nitrating a metal oxide (say MgO) is to treat with NO2 gas (caution: toxic and corrosive), as to quote Wikipedia on Nitrogen dioxide:

NO2 is used to generate anhydrous metal nitrates from the oxides:[10]
$\ce{MO + 3 NO2 -&gt; M(NO3)2 + NO}$

where heating mix of Cu, KNO3 and H2SO4 will generate NO2.
A safer path to nitrating, starts with the introduction of the powerful  radicals, like the hydroxyl radical (•OH). The latter can transform commonly available nitrate salts  (like KNO3) to the nitrate radical (•NO3) especially in the presence of an acid catalyst per the reaction:
$\ce{•OH + NO3- &lt;=&gt; OH- + •NO3}$
A supporting source reference, Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol. Per the paper's abstract:

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere.

Also, see The Reactivity of the Nitrate Radical (•NO3) in Aqueous and Organic Solutions, by
Stephen P. Mezyk, Thomas D. Cullen, Kimberly A. Rickman and Bruce J. Mincher, per the abstract to quote:

ABSTRACT
Rate constants for the nitrate (•NO3) radical reaction with alcohols, alkanes, alkenes, and several aromatic compounds were measured in aqueous and tert‐butanol solution for comparison to aqueous and acetonitrile values from the literature. The measured trends provide insight into the reactions of the •NO3 radical in various media. The reaction with alcohols primarily consists of hydrogen‐atom abstraction from the alpha‐hydroxy position and is faster in solvents of lower polarity where the diffusivity of the radical is greater. Alkenes react faster than alkanes, and their rate constants are also faster in nonpolar solution. The situation is reversed for the nitrate radical reaction with the aromatic compounds, where the rate constants in tert‐butanol are slower. This is attributed to the need to solvate the NO3− anion and corresponding tropylium cation produced by the •NO3 radical electron transfer reaction. A linear correlation was found between measured rate constants in water and acetonitrile, which can be used to estimate aqueous nitrate radical rate constants for compounds having low water solubility.

With respect to the hydroxyl radical, the UV photolysis of a photocatalyst like TiO2 in aqueous KNO3, could be a path to the powerful, but very transient, hydroxyl and nitrate radicals.