  HR8799 is a benchmark system for direct imaging studies. It hosts two debris
belts, which lie internally and externally to four giant planets. This paper
considers how the four known planets and a possible fifth planet, interact with
the external population of debris through N-body simulations. We find that when
only the known planets are included, the inner edge of the outer belt predicted
by our simulations is much closer to the outermost planet than recent ALMA
observations suggest. We subsequently include a fifth planet in our simulations
with a range of masses and semi-major axes, which is external to the outermost
known planet. We find that a fifth planet with a mass and semi-major axis of
0.1$\mathrm{M_J}$ and 138au predicts an outer belt that agrees well with ALMA
observations, whilst remaining stable for the lifetime of HR8799 and lying
below current direct imaging detection thresholds. We also consider whether
inward scattering of material from the outer belt can input a significant
amount of mass into the inner belt. We find that for the current age of HR8799,
only $\sim$1\% of the mass loss rate of the inner disk can be replenished by
inward scattering. However we find that the higher rate of inward scattering
during the first $\sim$10Myr of HR8799 would be expected to cause warm dust
emission at a level similar to that currently observed, which may provide an
explanation for such bright emission in other systems at $\sim10$Myr ages.
