Yes, it is possible to import and SRT or VTT subtitle file into your video. Below are the steps.
In Sonix, open the video you want to add an existing SRT file to (if you haven't uploaded the video to Sonix, you'll need to do that first.)
Click on the "Timecodes" dropdown in the top navigation
Choose "Import SRT file or existing transcript" (See image below)
Copy & paste the SRT file into the field provided
Wait as we automatically align your SRT file with you video 🎉
This video does a great job showing how you can upload your video file with a transcript (SRT or VTT file): Upload file with existing transcript