Choose outbound context for at queue

I have a system where I want to choose which outbound context the queue calls out to public phones.
I can’t use outbound routes and a pre-number as it is the same for everyone (we use no pre-numbers but custom-context).

If I create an queue manually (outside FreePBX) I can set the members to be eg:
Local/[email protected]/n,0
But FreePBX won’t accept it on the queues pages and if I edit the MySQL directly it dosn’t work.

Can some one help me?!