This is a onetime activity i have the evtx files and want to upload to splunk for analysis
Have a look at this page in the docs:
http://docs.splunk.com/Documentation/Splunk/latest/Data/MonitorWindowseventlogdata#Constraints
That has more details on how to import remote evtx logs any specific details on how to import already exported logs i have the evtx files for investigation?
The section I linked to is all about ingesting exported evtx files. As it is explained there: use a windows box and create a file monitor input that monitors the folder that holds the evtx files. Apart from some of the constraints mentioned in the docs, it is as simple as that.