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.
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