Can you turn your sketch into lineart, if so then how?
Yes. You can use the filter Extracting Lines (this works well, use the little black triangle sliders to adjust the filter before hitting OK), or you can set a black-on-white sketch to Multiply blend mode to make the white areas transparent.
If you are hand-painting colours underneath, either of these should work well.
For Bucket fills later (requires absolutely clean transparent areas), I find both the Extracting Lines filter and Multiply blend mode work best with a pure-black and white line image - either in your scanner software settings, converted later (I use IrfanView), or converting to a 1-bit layer in FireAlpaca (not so good with thin lines).
Note that a pure black and white image will often result in some aliasing and loss of line quality. The best results will probably come from scanning in greyscale or colour, using the Extracting Lines filter, and hand-colouring in the layers below.