Could you put it on, mark the positions of the add-ons, then take it off and sew them? Or temporarily glue them on while wearing it, then sew them?
Past Cosplay: Death of the Endless (Sandman), Trinity (The Matrix), Austria (Axis Powers Hetalia), Orihara Izaya (Durarara!!), Mey-Rin (Black Butler)
Current Cosplay: Dodo Chaplet (Doctor Who)
Future Cosplay: Dalek dress, Mags (both also Doctor Who)
For cosplay photos please visit my tumblr