You can mix Razor and ASPX in one project, BUT you can not reuse the Master page from ASPX as a Razor layout page (at least not without some tricks AFAIK).
So depending on the size of the already existing application and the amount of additions it may makes sense to convert the existing pages to Razor or continue to use ASPX.
For maintainability reasons I personally would NOT mix the two in the same application.
UPDATE after comment:
1000 screens is definitely NOT mid-size anymore IMO, BUT if you are not using layouts and/or Master pages your may are really best of "mixing them" to benefit from the better/easier syntax and perfromance for new pages, but no requirement to adapt the old ones righ away. Old pages can be adaped one by one, when they are changed for other reasons...
Your team shouldn't have a big problem getting into Razor, as it is very near to "normal programming" and you only need to learn very few syntax "specialities"...