Avid MC in fact _will_ handle that for you. It has had Mix&Match for many versions.
If you link to the 25fps clips in your 29.97 project and transcode, the framerate conversion will be baked in. That is not really the recommended method. Better workflow is to open the 25p clips in a 25p project, transcode the clips at 25p, then open the bin with the resulting 25p clips in a 29.97 timeline. You will notice a green dot, which is a motion adapter that keeps the material running at 100% realtime, enforcing a framerate conversion. You can adjust this, or even promote it to a TimeWarp, which gives you a whole lot more control over how the framerate conversion will look.
That said, a framerate conversion from 25p to 29.97p is not an easy one – not for any software. If quality is paramount, it could be a wiser choice to have everything converted through an Alchemist. But you can also first try the in-MC method and see if that works for you. If not, pull the selects and have them converted and match them back in.