Hi, I think that doing the conversion piecemeal is probably more effort than you would gain. I would simply take the time to do the conversion and then switch once you are ready.
Having said that, it may be possible - though likely harder if you are using https/http2 with CORS. uibuilder should render OK in an iFrame as long as your browser doesn't decide it doesn't like it.
But also, don't forget that Dashboard is a single-page app (and many uibuilder apps are as well though they don't have to be). Which means that if you were to embed Dashboard as an iFrame in a uibuilder page, you'd get the whole thing, you can't just have a single tab because in reality everything is just one enormous page, just with bits cleverly hidden).
If you did want to embed uibuilder pages into Dashboard until you were ready, and if you are using something like VueJS, I would look to create each "page" as a component so that you could start with them being displayed on a separate uibuilder html page but later combined into a single-page app. That is relatively easy to do with Vue though it takes a little more thought up-front. Vue also lets you unload component instances dynamically if you need to save resources but still keep the advantages of an SPA.
