how can I create a PO with item category L
but with a material without a bill of material?
If you send the 'sample material' out in MB1B + 541 + refering to your SC vendor, a material document will be created and you can track the process.
You can also make a delivery for this material movement if you want something different from 'simple' material document.
But creating a PO only because of this seems to be a very 'unnatural' solution to me.
(If you send out subcon-components to the same vendor independently from this and you want to do this with PO reference you can add this new component to the component list on your PO but I think it will cause more confusion than simple MB1B 541)