Running a T-SQL job under another user
There is no Run As proxy for T-SQL Job steps in SQL Server Agent.  You could upvote this with the chance that one day it will be filled:  

https://connect.microsoft.com/SQLServer/feedback/details/632955/enable-sql-agent-proxies-to-run-t-sql-job-steps

The current way to get a T-SQL command to run with a proxy is to run it as an Operating System Command which does support a proxy user (instead of T-SQL which does not) and run the task either using SQLCMD.EXE or PowerShell to execute the code.