I don't think this is what you want to do, though the specific answer to how to alias a sourcetype is given later. It seems to me that you simply want to specify a sourcetype for a set of input files. Normally, you can simply specify one when you create the input, either in the Manager GUI, or with sourcetype = mysourcetype in inputs.conf, or with a sourcetype stanza based on source in props.conf.
If you were using a Splunk forwarder that would be it. If not, you may have to use a TRANSFORM stanza to modify/set the sourcetype at index time, much as with host names: http://www.splunk.com/base/Documentation/latest/Admin/Overridedefaulthostassignments
You can rename sourcetypes in 4.x. props.conf.spec says:
rename = <string>
* Renames <sourcetype> as <string>
* With renaming, you can search for the sourcetype with sourcetype=<string>
* To search for the original sourcetype without renaming, use the field _sourcetype
therefore, for example:
[myoldsourcetype]
rename = mynewsourcetype
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