Yes, go to Lookup | Account | Query Builder.
Click the ContactID in the right pane, select Create Local Join, select Contact LeadSource in the drop down, and select ContactID as the ID.
Now, when you press OK - you will find the Contact Lead Source (and the lead source table) now available for selection.
You can pretty much join anything to anything via this Local Join. SLX doesn't display all possible combinations simple because the list would be too wieldy to use.
Regards
Mike