We find that a negative charge of an armchair single-walled carbon nanotube can significantly enhance migration of a carbon adatom on the CNT external surface along the direction of the tube axis. The main effect of charging is the stronger binding of the adatom to the CNT, which in turn increases the migration distance a few orders of magnitude due to stronger adsorption and consequent longer lifetime of adatom before desorption. These results support hypothesis that the carbon atoms feedstock for the SWCNT growth in the volume of arc plasma could be significantly supplied by the adsorption of the carbon atoms on the CNT surface, aiding to the flux to the metal catalyst surface.