I don't know about in vanilla RTW, but in BI you can.

Quote Originally Posted by descr_sm_factions.txt
faction slave
culture roman
symbol models_strat/symbol_slaves.CAS
rebel_symbol models_strat/symbol_slaves.CAS
primary_colour red 155, green 145, blue 120
secondary_colour red 90, green 75, blue 40
loading_logo loading_screen/symbols/symbol128_slaves.tga
standard_index 17
logo_index 246 ; Index into strat3.rsd
small_logo_index 260 ; Index into shared.rsd
triumph_value 5
intro_movie
victory_movie fmv/victory/slave_win.wmv
defeat_movie fmv/lose/rebels_eliminated.wmv
death_movie fmv/lose/rebels_eliminated.wmv
custom_battle_availability no
can_sap yes
prefers_naval_invasions no
Just change the line in bold to be yes.