To do this you must first, destroy your Frostwolf insignia, then do quests in Hillsbrad Foothills until you get the quest "infiltration". This will disguise you as a SI: 7 Agent. Then you can talk to the npc at the Stormpike AV enterance and ask for a new insignia.
1) Destroy your Horde insignia
2) Quest in Hillsbrad Foothills until you get Infiltration from the banshee in Tarren Mill
Note: Very long chain, starts at Southpoint Gate with "Welcome to the Machine"
Questecimation - WoWWiki - Your guide to the World of Warcraft
3) Talk to Lt. Hagger at the Alliance enterance, ask for a new insignia.
CONFIRMED: Does teleport you to Stormpike GY.
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