I'm trying to import a vector image from Inkscape into cinema 4d. I want to make the 2d image into a 3d image and use it within a project I already started. It doesn't look like Inkscape gives the ability to save an image as a ".ai" file type. I don't own Illustrator or Photoshop. Is there another way to do this? I've searched around and can't seem to find a solution.

Welcome to InkscapeForum!

So apparently Cinema 4d doesn't accept SVG? Is AI the only format the Cinema 4d accepts? If that's the case, I'd say you're stuck. But I'd be surprised if AI is the only format it accepts. Perhaps you could compare the list of what it accepts, with the list of what Inkscape can produce.

Might depend on which version of C4D you use but .eps could be a solution. If you have some, so to say, simple vector drawing, .eps should do the job but bare in mind that some effects are not quite nicely converted from .svg to .eps.

p.s.
If I recall it properly, .eps could be simply dragged into the viewport, otherwise File/Merge.