Jen Wolke from Blue Modus has an article about combining Portal Engine and MVC that might be worth reading through.
Honestly though, I would only recommend this approach as a last resort.
The technological changes between Portal Engine + Page Templates/Layouts/Web Parts and MVC + Page Builder is vast. In addition, Xperience 13 is already promoting an ASP.NET Core first approach for projects.
By the time you finish your in-place migration and have invested heavily in Kentico 12 MVC on .NET Framework, the platform/community will be heavily integrated with .NET Core (more likely, .NET 5).
At the recent Kentico Xperience Connections conference, it was discussed that the Content Management application is also going to be re-built on .NET Core within the next 9-18 months.
If running Portal Engine and MVC side-by-side in Kentico 12 is your only option for a migration strategy, it is likely doable. However, it isn't a 'supported' solution in any way and I predict many hurdles on the road to completing the project. By the time you do complete it I would expect that you'll have heavily invested in compatibility code to make the side-by-side solution work - time would could have been spent migrating to Xperience 13 on .NET Core.