To be honest I'm not sure how that would work ..

However, in the equipment folder there is a file called WeaponModDB.ini

That file controls the weaknesses to various weapon types and therefore shields strengths in terms of types (positron, graviton etc) as you describe.

You can add a custom type to that file, but you must add it to all the weapon entries on the file.

[WeaponType
nickname = W_Laser01
shield_mod = S_Graviton01, 0.800000
shield_mod = S_Graviton02, 0.800000
shield_mod = S_Graviton03, 0.800000
shield_mod = S_Molecular01, 1.000000
shield_mod = S_Molecular02, 1.000000
shield_mod = S_Molecular03, 1.000000
shield_mod = S_Positron01, 1.200000
shield_mod = S_Positron02, 1.200000
shield_mod = S_Positron03, 1.200000
shield_mod = MYSHIELDTYPE, 0.1 ; <----- custom shield type, highly resistant to lasers...

Then in your custom shield you'd do this ...

[ShieldGenerator
nickname = shield01_mark01_hf
ids_name = 263799
ids_info = 264799
DA_archetype = equipment\models\st\li_refractor_shield.3db
material_library = equipment\models\li_equip.mat
HP_child = HpConnect
hit_pts = 400
explosion_resistance = 0.500000
debris_type = debris_normal
parent_impulse = 20
child_impulse = 80
volume = 0.000000
mass = 10
regeneration_rate = 14.500000
max_capacity = 654
toughness = 6.500000
hp_type = hp_elite_shield_special_1
offline_rebuild_time = 12
offline_threshold = 0.150000
constant_power_draw = 0
rebuild_power_draw = 10
;
SHIELD_TYPE = MYSHIELDTYPE <------ Put your custom shield type here.
;
shield_collapse_sound = shield_offline
shield_rebuilt_sound = shield_rebuilt
shield_hit_effects = 0, gf_li_shield01
shield_hit_effects = 100, gf_li_shield02
shield_hit_effects = 500, gf_li_shield03
separation_explosion = sever_debris
LODranges = 0, 20
lootable = true

This now means that if your shield was shot by W_Laser01, then there would hardly be any damage due to the toughness you added with the above code.

At least thats what I believe, I haven't tested my own idea but at least this gets you on the right track to creating a shield that covers against all forms of damage against a shield.