Hello
As far as I know, it is not possible to create a source list valid for MRP without an info record. In my internal test systems, when I try to create a source list for a vendor without info record, an error message is triggered and this message is hardcoded as error, so you should not be able to change it in customizing.
Did you manage to create such source list on your system? If so, how did you create it?
The following WIKI provides more details about the source determination on MRP using a source list:
MRP Source Determination for External Procurement - ERP Manufacturing (PP) - SCN Wiki
If you don't want to follow this logic and you cannot create an info record, maybe you can try to use BAdI MD_MODIFY_SOURCE to select a vendor. I have never tested it to select a vendor without info record, but it may be possible.
Regards
Caetano