Material creation using procurement type as 'X'

Hi,

My requirement is to change the procurement type to X so that my product can either be supplied by my DC or i can raise third party PO when required.

When i try to create an article using MM41. Procurement Type defaults to F and the field is disabled hence i can not change it to X.

How should i go about changing the procurement type to X

Thanks in advance,

Reenav