I have two Illustrator files I would like to use in Animate to create a shape tween. Is it possible to import/convert these files to a shape so the program will allow me to create a shape tween?
Shape Tweens work well on simple shapes.
It is not impossible to shape tween more complex shapes, but it often requires to actually break them down to simpler elements on multiple layers.
You can import or even just copy/paste vector artwork from Illustrator to Animate.
Then you will need to re-organise that by re-layering and breaking apart until it becomes controllable.
You cannot just feed Animate with any two vector images and expect to get satisfactory results by applying a Shape Tween between them.
In conclusion: You need to master the use of Shape Tweens first with shapes created in Animate to see for yourself how they behave and then you will be able to have a realistic judgement whether the two files in question could be shape tweened.
My impression from the way you have worded your question is that at the moment your expectations are unrealistic.
I think you've misunderstood my question. Say I have an Illustrator file with basic geometric shapes. Is it possible to import it (or its layers) into Animate as a Shape? Or do you have to build shapes in Animate in order for a shape tween to work?
Yes, certainly you can!
You can either use File>Import an AI file or you can copy/paste and the shapes will remain vector and intact.
To animate them, you may need to further re-organise them structurally, i.e. distribute to layers, remove group wrapping, convert to symbols, etc. but all this is technical stuff which is easily done.