usually perforce integration is to merge code from some revision of source branch to LATEST revision of target branch.
Is there a way to merge code to a specific earlier revision of target branch?
Of course, it's still needed to sync to latest revision for submitting the code.
I think you can sync to the revision you want to integrate to, then run an integrate -h (have revision). Should be one of the checkboxes in the integrate dialog. Or just do it from commandline.
Then once you've done the integrate and resolve, you'll likely have a sync to head and another resolve to go through, but that ought to get you were you want to go.