If you take a look at how Flutter is implemented, you'll see that at the bottommost layer (dart:ui), it is imperative. On top of that, we built the rendering/painting layers (pretty imperative) and then the widgets layer (reactive).
Each of these layers is public API and follows the same breaking change process as any other part of Flutter's public API, so with a bit of effort an alternative imperative framework could conceivably be built built on top of what's there. The downside is that since the widgets layer is reactive, you'd be giving up all those shiny widgets that are part of the SDK and need to build your own (or find a way to wrap what's in the SDK).
TL;DR it's entirely possible to create an imperative framework on top of Flutter's lower layers, re-using a lot of our existing code, but (as far as I know) such a thing doesn't exist today.
Each of these layers is public API and follows the same breaking change process as any other part of Flutter's public API, so with a bit of effort an alternative imperative framework could conceivably be built built on top of what's there. The downside is that since the widgets layer is reactive, you'd be giving up all those shiny widgets that are part of the SDK and need to build your own (or find a way to wrap what's in the SDK).
This (old, but still accurate) talk by hixie covers the layers in detail: https://www.youtube.com/watch?v=dkyY9WCGMi0
TL;DR it's entirely possible to create an imperative framework on top of Flutter's lower layers, re-using a lot of our existing code, but (as far as I know) such a thing doesn't exist today.