Bit of a newb question, I know.
I was wondering if it was possible to create keyframes in one software (ex. Inkscape), then import to Synfig and have it do the 'tweeners.
For example, I have frames A and B in Inkscape. Say that frame A is a circle, and frame B is a circle scaled up on both axis.
Is it possible to import these into Synfig and end up with a smooth, “scaling up” animation?
Hi and welcome here 
I would say yes and no in the same time.
Synfig uses modification of states of keyframe/waypoints in Animation Mode, but all these are relative to the initial position/state/value.
So you will not be able to use B even though you move the timeline.
But…
You can still import your B and use it as a reference for placement (as a background image) 
There is no direct way (indirect would consist of writing a script or even synfig code). Synfig stores shapes as list of bezier knots and animated shapes as lists of animated knots - not as animated list (or, in other words, as soon as your shape is animated it no longer contains any individual state as a whole).
This issue was one of the things i wanted to fix in Synfig. In fact, adding equal knot count frame-by-frame morphing wouldn’t be that hard. On the other hand, morphing through shapes with different knot count would require quite some effort, not only in actual coding, but also in designing a flexible system (unless you are ok to be stuck with uneditable mess).
Thanks for taking the time to reply!
It’s great to know that before I did any real work. It’s also nice to know that reference images can be used.
I can understand where the difficulty in implementing such a feature comes from, but it’s pretty nice to see that someone’s thought about this.
Again, thanks all!