There is no automated and straight forward way to properly convert bitmap images to vector images (SVG in this case). I suspect that you have simply "embedded" your bitmap image inside an SVG, which doesn't really make it vector. IcoMoon only works with vectors.
Bitmap images (such as PNGs) consist of pixels. Vectors on the other hand, are made of numbers, points, curves, etc. As a result, when you scale up a bitmap image, it gets "pixelated" while vector images can be scaled up without problem.
In order to properly convert a bitmap image to a vector, you will need to draw the lines/curves in a vector editing software such as Adobe Illustrator, Inkscape or Sketch.