Can I simulate the fusion reaction of proton and boron-11 at approximately 675 keV with FLUKA? If so, do I need to define additional information in input other than just defining proton beam and boron-11 target?
Reaction cross sections for protons in FLUKA come from parametrizations for general A,Z. For protons on 11B, the onset of the parametrized cross section is around 2.2 MeV, so if you put protons at 675 keV you will unfortunately not see them undergo nuclear reactions.
Incidentally, note that (unless arithmetic error here…),
Q=m_p+m_{11B}-m_{12C}=15.96\ \textrm{MeV},
so (at energies such that the FLUKA cross section is nonzero) the reaction would start at a highly excited 12C. It is unlikely that it would remain as such, i.e. you will start in the fusion “channel” but during relaxation other nucleons/fragments will be emitted and the reaction will proceed along another path. E.g. the alpha separation energy is around 7 MeV, so (p,alpha) will be among the likely processes, that is, (p,3alpha).