It sure is possible. All you will need is the reference entity file for the Advent starbase. Open it using the notepad (it should already be in text downloaded from forgetools) pick any one of the weapons that you are dissatisfied with. Then replace is with this
Weapon
WeaponType "Beam"
damageEnums
AttackType "CAPITALSHIP"
DamageAffectType "AFFECTS_SHIELDS_AND_HULL"
DamageApplyType "OVERTIME"
DamageType "ENERGY"
WeaponClassType "BEAM"
DamagePerBank:FRONT 105.000000
DamagePerBank:BACK 0.000000
DamagePerBank:LEFT 0.000000
DamagePerBank:RIGHT 0.000000
Range 4800.000000
PreBuffCooldownTime 7.000000
CanFireAtFighter FALSE
SynchronizedTargeting FALSE
PointStaggerDelay 0.500000
TravelSpeed 0.000000
Duration 4.000000
fireConstraintType "CanAlwaysFire"
WeaponEffects
weaponType "Beam"
burstCount 1
burstDelay 0.000000
muzzleEffectName "Weapon_PsiCapitalBeamHeavy_Muzzle"
muzzleSoundMinRespawnTime 0.100000
muzzleSounds
soundCount 1
sound ""
hitEffectName "Weapon_PsiCapitalBeamHeavy_Hit"
hitHullEffectSounds
soundCount 1
sound ""
hitShieldsEffectSounds
soundCount 1
sound ""
beamEffectSounds
soundCount 1
sound "WEAPON_PSICAPITALBEAMHEAVY_TRAVEL"
beamGlowTextureName "PsiCapitalBeam_Glow"
beamCoreTextureName "PsiCapitalBeam_Core"
beamWidth 30.000000
beamGlowColor ffffffff
beamCoreColor ffffffff
beamTilingRate 3.000000
This was taken from the Psibattleship entity so you would adjust the damage points and directions it fires as you feel appropriate as well as range and fire rate (PreBuffCooldownTime), just play around with it to see what suits it best. Depending on which Starbase weapon you decide to replace with this remember the fireConstraintType that weapon had as this determines when the beam will be available. At the moment it's set at CanAlwaysFire which means it will be available from the moment the starbase is built. Hope this helps 
