If you want to copy data with the collect-command you should also add informations to host, sourcetype and source.
Otherwise these fields get a splunk-internal name.
index="foo" | collect index="bar" host="bar1" source="bar2" sourcetype="bar3"
You can go into settings -> Indexes and create a new index. (FooBar) Then, you can use the collect command to move all the data to the new index.
If it's just for search time convenience, consider adding the same tag to each index as per this one...
Then, for cleanup purposes, create your new index, tag THAT index with the same tag, and redirect all indexing on the low-volume indexers to the new index. The data on the old indexes will roll off over time, cleaning up your system without you having to muck about with actually moving the old data.
Seems like you need to use collect command.
Refer to the following documentation
Refer to the video tutorial to understand the concept of summary indexing: https://www.splunk.com/view/SP-CAAACZW