You can't control dragonfire like that. It's a special hardcoded thing.
Anyway, I went and made a creature for you. Make a file in your object/raw folder called creature_sporesac.txt and stick this in it.
creature_sporesac
[OBJECT:CREATURE]
[CREATURE:SPORE_SAC]
[DESCRIPTION:A floating subterranean organism. When agitated, it violently explodes, scattering its spores and repulsing nearby creatures. It is sustained by nutrient-rich liquids, which it seeks out and absorbs with its hanging tendrils.]
[NAME:spore sac:spore sacs:spore sac]
[CASTE_NAME:spore sac:spore sacs:spore sac]
[CREATURE_TILE:'s'][COLOR:6:0:0]
[TRAPAVOID][FLIER]
[LARGE_ROAMING]
[NOBONES]
[BIOME:SUBTERRANEAN_CHASM]
[UNDERGROUND_DEPTH:2:3]
[FREQUENCY:25]
[CURIOUSBEAST_GUZZLER]
[POPULATION_NUMBER:100:300]
[CLUSTER_NUMBER:2:3]
[EXTRAVISION][NOBREATHE][NOSTUN][NONAUSEA][NOEMOTION]
[GRASSTRAMPLE:0]
[PREFSTRING:lazy drifting]
[PREFSTRING:explosive tendencies]
[BODY:BASIC_1PARTBODY_FLYING_HEAD_FLAG_THOUGHT:TWO_NO_CLAW_TENTACLES]
[USE_MATERIAL_TEMPLATE:SKIN:STRUCTURAL_PLANT_TEMPLATE]
[USE_MATERIAL_TEMPLATE:SPORES:PLANT_POWDER_TEMPLATE]
[STATE_NAME_ADJ:ALL:spore sac spores]
[STATE_COLOR:ALL:ASH_GRAY]
[DISPLAY_COLOR:7:0:0]
[PREFIX:NONE]
[SYNDROME]
[SYN_NAME:spore sac mortality]
[SYN_AFFECTED_CREATURE:SPORE_SAC:ALL]
[SYN_CONTACT]
[CE_BLEEDING:SEV:1000000:PROB:100:BP:BY_CATEGORY:ALL:ALL:START:0:PEAK:3]
[CE_PARALYSIS:SEV:100:PROB:100:BP:BY_CATEGORY:ALL:ALL:START:0:PEAK:3]
[BLOOD:LOCAL_CREATURE_MAT:SPORES:POWDER]
[TISSUE:SPORES]
[TISSUE_NAME:spores:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:SPORES]
[TISSUE_MAT_STATE:POWDER]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:10]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LEAKS]
[VASCULAR:10]
[TISSUE_LAYER:BY_CATEGORY:BODY:SPORES]
[TISSUE:SKIN]
[TISSUE_NAME:skin:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:STRUCTURAL_PLANT_TEMPLATE]
[TISSUE_MAT_STATE:SOLID]
[STRUCTURAL]
[MUSCULAR]
[RELATIVE_THICKNESS:3]
[CONNECTS]
[VASCULAR:2]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:ALL:SKIN]
[MATERIAL_BREATH_ATTACK:LOCAL_CREATURE_MAT:SPORES:UNDIRECTED_DUST]
[SPEED:3200]
[BODY_SIZE:0:0:45000]
[ALL_ACTIVE]
[NO_SLEEP]
[NO_DIZZINESS]
[NO_FEVERS]
[SET_TL_GROUP:BY_CATEGORY:ALL:SKIN]
[TL_COLOR_MODIFIER:BUFF:1]
[TLCM_NOUN:skin:SINGULAR]
When it gets next to a creature, it explodes and sends everything near it flying. It dies soon afterward, along with other nearby spore sacs. I had to put make them spawn in clusters, because creatures with only a couple body parts tend to die really easily and you can't get this sort of effect to occur on death, and they're slow, so they have to attack in groups if you want the explosion thing to happen before they die.
After the "explosion," they stop moving and quickly bleed out, which simulates a kind of "ruptured" effect.
Also they seek out and steal your booze, which should help get them to your dwarves.