Maybe a stupid question, but is it possible to have a workflow "add button" display on a sub tab instead of up along the top bar with the other buttons?

No but depending on the action you can use an inline html field to add a button that goes to a url or other record

Yap, not possible by Workflow, but may be possible by a script

It depends on what you want the button to do. You can create a button from a standard form "field", and put that within reason anywhere you want. Its an inline HTML field. What the button does may need a script or a little bit of HTML for a link to a search. I created this for a customer (colour was their requirement). Default Value was this: <button type="button" onclick=" nlExtOpenWindow('ENTER SEARCH URL HERE','search', 1400, 600, null, 'yes', 'Search Details'); " " style="border-radius: 4px; font-size:14px; font-weight:600; background-color: #FE835F !important; padding:5px">Show Quote Lines for this Item</button>

I need it to trigger a workflow

For this you would need a script that would use the workflow.initiate() object https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_4344303916.html

yeah i figured that but it was worth a check ! lol