It's quite simple. Open FacilityTypes.xml and search for <ubMilitiaTrainersAllowed>. You will find facilities that allow militia training. Look for one that somewhat matches the setting. For example on WF maps we have this facility:
<FACILITYTYPE>
<ubIndex>21</ubIndex>
<szFacilityName>Rebel Hideout</szFacilityName>
<szFacilityShortName>Rebel Hideout</szFacilityShortName>
<ubMilitiaTrainersAllowed>2</ubMilitiaTrainersAllowed>
</FACILITYTYPE>
You could also create a new facility type. Remember the <ubIndex> of the facility type that you want to use. In Facilities.xml create a new entry like this:
<FACILITY>
<SectorGrid>A10</SectorGrid>
<FacilityType>21</FacilityType>
<ubHidden>1</ubHidden>
</FACILITY>
The above is the definition for facility type 21 "Rebel Hideout" in sector A10. You need to use the facility type that you selected from your FacilityTypes.xml. <ubHidden> means that it's initially hidden until you explore the sector.