Project related Purchase orders

Hi,

Let's say I need to enter a new purchase order that is project related. I start by clicking new PO, I enter the vendor ID, and in the general fast tab I enter the project and click OK, then I can enter by line item the project category,....

Now if I start by clicking new PO, I enter the vendor ID, and in the general fast tab I MISS  to enter the project and click OK, then I can NOT  enter by line item a project ID or  project category,.... they are disabled fields.

Is there any way to be able to make the project ID field editable so I can enter a value. The only choice I have now is to enter a new PO from scratch.

Thanks

Sam