Sorry, no. There's at least two methods of encoding subtitles/captions. One has the position encoded with the caption and one doesn't. For the one that doesn't, they will be displayed centered near the bottom of the screen. For the one that does, they will get displayed wherever they're requested to be, so if you're getting captions other than centered at the bottom you can bet that it's part of the caption itself. And then there's captions that are part of the video itself (often foreign language translations). There's no way to affect those in any way. It's also possible that the app itself is handling captions and Roku wouldn't have any control over what it does.
Roku could probably add an option to always display captions at the user's preferred location (probably top or bottom) and override any position specified in the caption. There's a place to suggest features:
Suggest a Feature | Roku Community | Roku
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