I think that you can use the appendcols command to do that.
To have the resulting table you need you have to pipe the search that produce the first result without user and then you use appendcols [ ] .in the square brakets you put the search that will produce only the field user from WSA sourcetype.
At the end you pipe all and you can use table command to display what you wanted. just like this:
search from ASA sourcetype | appendcols [subsearch from WSA] |table timestamp scr_ip count user