This is contingent on the format that you're exporting... many subtitles formats make this difficult if not impossible. I recently finished the restoration of a documentary from the 1980's where the client wanted some of the subtitles to reposition vertically. I did finally make this work for both dvd and online streaming, but it wasn't easy. There's a windows program https://www.nikse.dk/subtitleedit that makes this possible with certain formats but it requires thorough testing. If you're working on a mac, there is an online version.
If you want some help with this, post back with some more detail about what you're trying to do and I'll try and dig in to my memory and see if I can help. Stan Jones is the expert on this stuff so hopefully he see this post and help.