First I am going to provide a word of caution:
DO NOT Pour a spoutfull of powder down the barrel of a muzzle loader. Always transfer directly from the flask to a separate measure. That way you are only holding one charge of powder that may catch on a lingering spark at your breech. Follow up firing is rare, but I recommend that you load in safe manner.
Loading from a flask or horn is a practice that is not allowed on the traditional range where I shoot.
I recommend that you look at the powder flasks that pour directly into a removable measure. Deer Creek sells two variations and Track of the Wolf also has some.
Yes, the threads are standard. Use your smallest spout to pour out a load into your separate measure.