I haven't used ERA5 but since STILT is a superset of HYSPLIT tools, most meteorology that is compatible with HYSPLIT can be used with STILT. Beyond that, converting data to/from HYSPLIT's ARL format isn't really within STILT's scope. The easiest approach is likely the era52arl conversion utility maintained by NOAA ARL, and the HYSPLIT community forum would be a good place to get support.

NOAA ARL makes other meteorology products available (e.g. GFS and HRRR) that you can use out of the box, which might be easier than fighting with the converters.