Can I carry slush powder as a carry-on on a flight?

Sodium polyacrylate is not considered hazardous. It is not flammable, it is not explosive, it is not reactive, and is not poisonous. The worst it can do is irritate skin and mucosa. For these reasons you should be allowed to transport it on a flight in your hold or carry-on luggage. Note however that carry-on liquid and gels limitations will apply if you plan on transporting it in its gel form. Don't forget that airport security has the last words when it comes to allowing items in carry-on luggage or not. If they deem sodium polyacrylate to be hazardous they will seize and dispose of it.

Now, you might have to pay custom duties on sodium polyacrylate imports upon landing in Nepal, depending on whether it is subject to duties and the imported quantities. At a first glance, the Nepal Department of Customs site does not list it as prohibited:

Banned, Quantitative Restriction and Permission on Imports

What are the Prohibited/ banned goods for import

• Health hazardable narcotic intoxicating goods such as hashish, heroin, opium, muffin etc.
• Beef
• Hazardous ouzo dyes as specified by the Government of Nepal through publishing the notice in Nepal gazette time to time
• Plastics scrap and bags and sheets of plastics below the 20 micron thickness
• High carbon disposing incandescent light bulbs
• Goods prohibited/ banned by other existing laws

And stacklector or combined harvester with baler

What are the goods to be imported under license or permission letter?

• Narcotic and psychotropic medicine and raw-chemical for those medicine on the recommendation of the Ministry of Home Affairs.
• Arms and ammunitions, explosive substance, materials required to produce explosive substance, guns and bullet for gun caps except paper and other explosive substance, arms and ammunitions on the recommendation of the Ministry of Home Affairs.
• Radio equipment such as wireless, walkie-talkie, transmission receiver, link radio equipment etc., and similar kind of radio equipment such as transmission of words, dialogue, scene and statistics on the recommendation of the Ministry of Communication

You best bet would be to contact Nepales customs directly:

Department of Customs

Tripureshwor, Kathmandu
Tel : 01-4259861
Fax: 4259808
E-mail: csd@customs.gov.np

If you do, don't forget to mention the reason why you need to import sodium polyacrylate, showing proofs that you are attending to the conference and are organising a workshop.