well this is easy. add "SquadPsiMineLayer" to the 3rd strikecraft line in the advent carrier. heres an example:
squadTypeEntityDef:0 "SquadPsiCombat"
squadAntiMatterCost:0 0.000000
squadTypeEntityDef:1 "SquadPsiBomber"
squadAntiMatterCost:1 0.000000
squadTypeEntityDef:2 "SquadPsiMineLayer"
squadAntiMatterCost:2 0.000000
squadTypeEntityDef:3 ""
squadAntiMatterCost:3 0.000000
NumSoundsFor:ONATTACKORDERISSUED 3
SoundID "CAPITALSHIP_PSICARRIER_ONATTACKORDERISSUED_0"
SoundID "CAPITALSHIP_PSICARRIER_ONATTACKORDERISSUED_1"
SoundID "CAPITALSHIP_PSICARRIER_ONATTACKORDERISSUED_2"
NumSoundsFor:ONCREATION 1
SoundID "CAPITALSHIP_PSICARRIER_ONCREATION"
NumSoundsFor:ONGENERALORDERISSUED 3
SoundID "CAPITALSHIP_PSICARRIER_ONGENERALORDERISSUED_0"
SoundID "CAPITALSHIP_PSICARRIER_ONGENERALORDERISSUED_1"
SoundID "CAPITALSHIP_PSICARRIER_ONGENERALORDERISSUED_2"
NumSoundsFor:ONSELECTED 3
SoundID "CAPITALSHIP_PSICARRIER_ONSELECTED_0"
SoundID "CAPITALSHIP_PSICARRIER_ONSELECTED_1"
SoundID "CAPITALSHIP_PSICARRIER_ONSELECTED_2"
NumSoundsFor:ONSTARTPHASEJUMP 1
SoundID "CAPITALSHIP_PSICARRIER_ONSTARTPHASEJUMP_0"
MeshNameInfoCount 1
MeshNameInfo
meshName "CapitalShip_PsiCarrier"
criteriaType "None"
meshNameIncreasedEffectName ""
meshNameDecreasedEffectName ""
ExhaustParticleSystemName "Exhaust_PsiCarrier"
ExplosionName "CapitalShip0"
CommandPoints
StartValue 2.000000
ValueIncreasePerLevel 0.500000
weaponCooldownDecreasePerc
StartValue 0.000000
ValueIncreasePerLevel 0.020000
weaponDamageIncreasePerc
StartValue 0.000000
ValueIncreasePerLevel 0.060000
HyperspaceChargingSoundID "HYPERSPACE_CHARGEUP"
HyperspaceTravelSoundID "HYPERSPACE_TRAVEL"
EngineSoundID "ENGINE_PSICAPITALSHIP"
formationRank 2
minShadow 0.000000
maxShadow 0.500000