There was a conversion on
www.ps3e.com, but the webpage is currently down. You might consider turning a normal rat into a magical beast and adding the powers from 2nd ed, like maybe something like this:
Rat
Tiny Magical Beast
Hit Dice: 1d10 (5 hp)
Initiative: +2 (Dex)
Speed: 15 ft., climb 15 ft.
AC: 14 (+2 size, +2 Dex)
Attacks: Bite +5 melee
Damage: Bite 1d3-4
Face/Reach: 2 1/2 ft. by 2 1/2 ft./0 ft.
Special Qualities: Scent, Hive-mind
Saves: Fort +2, Ref +4, Will +1
Abilities: Str 2, Dex 15, Con 10, Int 2, Wis 12, Cha 10
Skills: Balance +10, Climb +12, Hide +18, Move Silently +10
Feats: Weapon Finesse (bite)
Climate/Terrain: Any land and underground
Organization: Swarm (10-100)
Challenge Rating: ???
Treasure: None
Alignment: Always neutral evil
Advancement: See text
Hive-mind (Su): Cranium rats are of sepearate and of one mind all at once. The more cranium rats are present, the greater the intelligence of the rats is. Each cranium rat has a telepathic connection with all the other rats within 10 ft. For every 5 rats, add an extra point of Intelligence to the score for the rats. For 4 or less cranium rats, the Int score is equal to that of the normal rat (2). Adding one more rat gives it an Int of 3. For every 5 more rats, an additional 1 is added to this score. As the intelligence grows, so does the pack's powers.
Int score Ability
7 or less Standard rat
8 spell-like abilities mimicing total of 1 wizard spell levels
9 spell-like abilities mimicing total of 2 wizard spell levels
10 Mind blast, 1 every 3 rounds, as mind flayer
11 spell-like abilities mimicing total of 3 wizard spell levels
12 spell-like abilities mimicing total of 4 wizard spell levels
13 Mind blast, 1 every rounds, as mind flayer
14 spell-like abilities mimicing total of 5 wizard spell levels
15 spell-like abilities mimicing total of 6 wizard spell levels
16 Mind blast, 1 every round, as mind flayer
17 Gas immunity
18 Poison Immunity
19 SR 13
20 SR 19
21 SR 25
Also, treat the pack as one entity when it comes to HD affected by an area-affecting damage spell. For example, if a 30-rat pack is hit by an 8 HD fireball, only 8 are killed if the pack's Reflex save is failed, or 4 if it is successful. Also, treat the rat's saves as coming from a Magical beast equal to the pack's Intelligence.
A cranium rat pack is quick to break telepathic bonds with an individual rat who is under the influence of an mind-influencing affect, so each rat must be so dealt with individuallly. However, killing the members of a pack reduces it's intelligence according to the current number of members.
So there's a fairly rough conversion. You might also want to ask about others' interpretation in the Creature Catalog forums.