You can build General citizens, which will give you legions, but they cannot be stationed or placed on ships until you have researched Planetary Invasion.  Once this is researched, you'll have access to the ship components necessary to build troop ships (there is one auto-designed for you on a cargo hull, if you don't like designing your own ships).

Militarization allows the General citizen, which provides 5 legions (and can be upgraded for 5 more).  The Station Garrison planetary project and Transport modules are unlocked with Planetary Invasion.  I believe if you gain a transport ship (through scavenging, trading, or the Bazaar) you can still load that ship with legions you have from Generals, without the PI tech, but you cannot build those ships yourself until the research.