The sourcetype specifier is a type of regex, so you could make the field alias something like (sourcetype1)|(sourcetype2) , or if you truly want it to apply to all sourcetypes (probably not, except if you set the alias to only be shared in a specific app), it should work with e.g. * .
Edit:
Scratch that, it is not a regex for sourcetypes, see http://docs.splunk.com/Documentation/Splunk/6.2.3/Admin/Propsconf
<spec> can be:
1. <sourcetype>, the source type of an event.
2. host::<host>, where <host> is the host, or host-matching pattern, for an event.
3. source::<source>, where <source> is the source, or source-matching pattern, for an event.
In that case I don't think there's a solution using aliases, unless your sourcetypes match your sources in some predictable way (in which case you can use wildcards in your fieldalias stanza).
So in short, yes, you need to create an alias for each sourcetype if you want the same alias for different sourcetypes
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