Fanfiction.net doesn’t allow downloading from its archive, and Archive of Our Own can’t import stories from there.
So this is a fairly quick workaround, though you do have to do the chaptering yourself. Get yourself two tabs, FFnet and AO3. On the FFnet tab, go to your Doc Manager. Note: You need your story’s chapters in the Doc Manager, so if they’re not there, just go to your story and export them to the DM.
FFnet: open the chapter in the Doc Manager editor.
AO3: go to your Dashboard and start a New Work.
fill in the ratings, pairings, tags, warnings, etc et al.
you can copy and paste your author notes from the FFnet editor.
AO3: down in the Work Text area, switch to Rich Text.
FFnet: select the body of your chapter in the editor, and copy.
NOTE: if you use center tags, these will not stick. go to each line (or block) that is supposed to be centered, select it, and press the Center Text icon. this should lock it in.
AO3: preview. if it’s messed up, edit. if not, post.
Repeat for each chapter.