Targeting MSX with job on Multi-Server Administration
Based on my understanding, the master server (MSX) cannot be a target server (TSX) that receives jobs from another master server i.e You cannot list one MSX server into another MSX, you have to defect it and make sure it is not enlisted with any other master server.
Its a simple two-level hierarchy: a server is a master server, a target server, or neither. Cannot be both at the same time.
FROM BOL :

When creating a multiserver environment, consider the following:

Each target server reports to only one master server. You must defect a target server from one master before you can enlist it into a different one.
The master and target servers must be running on the Windows NT 4.0 or Windows 2000 operating system.
When changing the name of a target server, you must defect it before changing the name and reenlist it after the change.
If you want to dismantle a multiserver configuration, you must defect all the target servers from the master server.


You may also want to read up : SQL Server Multiserver Administration: Master and Target Servers