Two options, you can edit a file manually, or download a mod. Link to mod can be found at post #8 of this thread:
https://forums.totalwar.org/vb/showthread.php?t=41604

The manual edit:
Find the file "descr_strat.txt" in ...\Rome - Total War\Data\world\maps\campaign\imperial_campaign

Cut the faction you want to play as from the section its under (either unlockable or nonplayable), and copy it into the playable section.