Can I change glass transition temperature (Tg) and impact resistance with additives?
The glass transition temperature depends on the cooling rate and molecular weight distribution and could be influenced by additives:

Most liquid plasticizers are low molecular weight organic materials with a glass transition temperature in the range of $125-225^0\text{C}$. When this kind of additives is added to an organic polymer, the result is a weight averaging of the glass transition temperature between that of the polymer and that of the plasticiser if they are miscible. The reduction of the the glass transition temperature of a polymer by the plasticiser is usually about   $5^0\text{C}$ per weight percent of plasticiser.

Impact modifiers ( a kind of additives) can affect the impact resistance of a polymer. Impact modifiers are key additives for
increasing flexibility and impact strength
to meet physical property requirements of
rigid polymers. All impact modifiers are elastomeric or
rubbery in nature, with a lower modulus
than the host polymer. The dispersed
rubber phase acts to absorb or dissipate the
energy of impact in order to stop craze or
crack propagation.