Since for whatever reason the engine will not let me add the NPC hab shell objects to the build menu, you will have to spawn them by console. I've taken the liberty of compiling all possible metal shell objects found in the game. Simply do; PlaceThere followed by the 8 digits within [STAT: XXXXYYYY]. EG: PlaceThere 00055BFD.

Hab shell list:

OpiExtAddOnHexLgShellGreeble01 [STAT:000179D5]
OpiExtAddOnHexLgShellGreeble02 [STAT:000179D6]
OpiExtAddOnHexSmShellGreeble01 [STAT:000179D7]
OpiExtAddOnHexSmShellGreeble02 [STAT:000179D4]
OpiExtAddOnHexSmShellTower01 [STAT:000179D9]
OpiExtAddOnHexSmShellTower02 [STAT:00068E8B]
OpiExtPodLgMidHexBase01 [STAT:0000B0E7]
OpiExtPodLgMidHexShell01 [STAT:0000B151]
OpiExtPodLgMidHexShellBot01 [STAT:0000B150]
OpiExtPodLgMidHexShellMid01 [STAT:0000B14F]
OpiExtPodLgMidHexShellTop01 [STAT:0000B14E]
OpiExtPodLgMidShell01 [STAT:0000B12E]
OpiExtPodMedMidShell01 [STAT:0000B130]
OpiExtPodMedWallExArchShell01 [STAT:0000B14C]
OpiExtPodMedWallExArchShell02 [STAT:0000B45A]
OpiExtPodMedWallExArchShellBot01 [STAT:0000B168]
OpiExtPodMedWallExArchShellMid01 [STAT:0000B169]
OpiExtPodMedWallExArchShellTop01 [STAT:0000B16A]
OpiExtPodMedWallExLgShell01 [STAT:0000B14B]
OpiExtPodMedWallExLgShell02 [STAT:0000B459]
OpiExtPodMedWallExLgShellBot01 [STAT:0000B154]
OpiExtPodMedWallExLgShellMid01 [STAT:0000B155]
OpiExtPodMedWallExLgShellTop01 [STAT:0000B156]
OpiExtPodMedWallExMedShell01 [STAT:0000B148]
OpiExtPodMedWallExMedShell02 [STAT:0000B458]
OpiExtPodMedWallExMedShellBot01 [STAT:0000B157]
OpiExtPodMedWallExMedShellMid01 [STAT:0000B158]
OpiExtPodMedWallExMedShellTop01 [STAT:0000B159]
OpiExtPodMedWallExWinShellTop04 [STAT:00029C40]
OpiExtPodMedWallExWinShellTop05 [STAT:0000B178]
OpiExtPodMedWallShell01 [STAT:0000B12F]
OpiExtPodMedWallShell02 [STAT:0000B149]
OpiExtPodMedWallShellBot01 [STAT:0000B15A]
OpiExtPodMedWallShellMid01 [STAT:0000B15B]
OpiExtPodMedWallShellTop01 [STAT:0000B15C]
OpiExtPodMedWallShellTop01a [STAT:0005059E]
OpiExtPodMedWallShellTop01b [STAT:0005059F]
OpiExtPodMedWallShellTop01c [STAT:000505A0]
OpiExtPodMedWallShellTop02 [STAT:0000B15D]
OpiExtPodMedWallShellTop02a [STAT:000781FC]
OpiExtPodSmMidHexShell01 [STAT:00055BFD]
OpiExtPodSmMidHexShellBot01 [STAT:000585E9]
OpiExtPodSmMidHexShellMid01 [STAT:000585E8]
OpiExtPodSmMidHexShellTop01 [STAT:000585E7]
OpiExtPodSmMidShell01 [STAT:0000B131]
OpiExtPodSmWallCorInShell01 [STAT:0000B132]
OpiExtPodSmWallExCorInShellBot01 [STAT:0000B17A]
OpiExtPodSmWallExCorInShellMid01 [STAT:0000B17B]
OpiExtPodSmWallExCorInShellTop01 [STAT:0000B17C]
OpiExtPodSmWallExLgShell01 [STAT:00055BFC]
OpiExtPodSmWallExLgShell02 [STAT:0000B45B]
OpiExtPodSmWallExLgShellBot01 [STAT:000585F0]
OpiExtPodSmWallExLgShellMid01 [STAT:000585F1]
OpiExtPodSmWallExLgShellTop01 [STAT:000585F2]
OpiExtPodSmWallExMedShell01 [STAT:00055BFA]
OpiExtPodSmWallExMedShell02 [STAT:00055BFB]
OpiExtPodSmWallExMedShell03 [STAT:0000B457]
OpiExtPodSmWallExMedShellBot01 [STAT:000585F3]
OpiExtPodSmWallExMedShellMid01 [STAT:000585F4]
OpiExtPodSmWallExMedShellTop01 [STAT:000585F5]
zAddOpiExtSolar02 [STAT:00024398]