This is a web version of my older python project which converts Stepmania/Etterna Charts (*.sm files) into FNF JSON files, which are compatible with the Psych-engine (as of v0.6.2)
Check it out here.
The program allows pretty much any chart in any *.sm file to be converted into Psych-engine. Just select the SM file, fill in the form and it should generate the resulting JSON file as a download.
Side Note: If you're converting a 4k chart instead of an 8k chart. Be sure to check the checkbox labelled "Flip chart" if you want bf (the player character) to get all the notes.
SM files can be generated either using Stepmania or using ArrowVortex, among other simfile editors.
Warning : This program will generate charts for any game mode (dance-single, dance-double, pump-single etc) although for Stepmania game modes that do not have 4 or 8 keys, the resulting json chart may not work as intended, so best stick to dance-single (only bf/opponent gets notes) or dance-double (for both bf and the opponent) charts alone. Currently does not support negBPM charts (They seem to get de-synced) but can handle bpm changes for the most part.
The older python project: SMToPsychFNF