Run workflow as service account

HI,

I'm trying to run one worklfow element in a larger workflow as a different user. Is this possible? I know you can right click -> start workflow as someone but that's one time and for the whole workflow. I want to be able to run one element as another user inside the main workflow. If this is possible I can't figure it out. I read this might be possible with policies but I couldn't figure that out either .

edit: Just to give further details, I want to be able to run the "delete or disable computer ad by name" workflow as a service account. I have this workflow in a larger decommission workflow.

thanks!

Chris