If all you want is more bases, disregarding the complex mission generation tables, you can add something like this under missionScripts in Piratez_Factions.rul (or better, make a submod with just this in a rule file):
missionScripts:
- type: alienBase
missionWeights:
0:
STR_ALIEN_BASE: 100
STR_ALIEN_BASE_ACADEMY: 100
# STR_ALIEN_BASE_GUILD: 100
# STR_ALIEN_BASE_CHURCH: 100
STR_MISSION_SWAY_GOVT: 100
firstMonth: 0 #8
useTable: false
executionOdds: 100 #5
startDelay: 1000
randomDelay: 1000 #30000
Tweak these numbers as desired (right now, it's set up for testing, so you see the base missions appear almost as soon as you start the game).
Note that proper alien bases only seem to come through infiltration (Sway Local Govt) missions, so you also get infiltrated. If you want space alien bases without infiltration, you'll need to do some extra work.