This just one example for the multiple space maps. With one example mode. Its spa3, or in game Space Kashyyyk.
The only problem is when I play any of the space maps is that the name changes do not appear on the weapons and units, otherwise it works.

addme
AddNewGameModes( sp_missionselect_listbox_contents, "spa3c%s_%s", {era_c = 1, mode_ass_c = 1,} )
AddNewGameModes( mp_missionselect_listbox_contents, "spa3c%s_%s", {era_c = 1, mode_ass_c = 1,} )

AddDownloadableContent("spa3c","spa3cc_ass",4)

mission
"spa3c_ass"

The only change to "spa3_ass.lua" was the added "dc:"

ReadDataFile("dc:SIDE\\cis.lvl",
"cis_inf_pilot",
"cis_inf_marine",
"cis_fly_fedlander_dome",
"cis_fly_droidfighter_sc",
"cis_fly_droidfighter_dome",
"cis_fly_greviousfighter",
"cis_fly_droidgunship",
"cis_fly_tridroidfighter")

Here's my overall progress with the space maps.
https://www.dropbox.com/sh/n35orbv5o91553l/5D3ltJ3W1-

Try this instead

addme
AddNewGameModes( sp_missionselect_listbox_contents, "spa3%s_%s", {era_c = 1, mode_ass_c = 1,} )
AddNewGameModes( mp_missionselect_listbox_contents, "spa3%s_%s", {era_c = 1, mode_ass_c = 1,} )

AddDownloadableContent("spa3","spa3c_ass",4)

mission
"spa3c_ass"