Hi Tom,
Thanks for the reply again!!!
As per my knowledge and understanding also, I don't think ABAP system has a provision to integrate with UME datasource by any means.
I do agree with what you said that users from ABAP datasource can be replicated to UME datasource using SPML service but as far as I know there is no direct authentication mechanism between ABAP system and UME datasource as ideally speaking this is seldom a requirement.
Just for the sake of knowledge, I am exploring this option and do have little hopes for such an integration 
Let's leave it to experts, in case if anyone has something else to say in this regard.
BR,
Anurag