Officially the law specifies that a motorcycle in Thailand has to be tested, and approved by the Thai Industry Standard organization (TIS). But I never seen or hear anything about this rule being enforced, and most serious motorcyclists (including me) trust the testing from Europe (ECE) and USA (DOT) over the Thai standard.
If you buy goods outside Thailand it's likely that you have to pay import tax, while some people seem to be able to receive goods without paying tax now-and-then. The amount of tax you need to pay depends on where the helmet was manufactured. As for some Japanese helmets you can get them free from import tax due to a free trade agreement.
We have not much experience of importing a motorcycle helmet from Hong Kong so I cannot tell you what the import duty will be. You probably can check the amount of import duty on a motorcycle helmet if you contact the Thai customs office.
It depends how many you want to import...
if you import one or two helmets (via shipping or courrier company), as 'personal effect', then you should not face any problem with customs, but duties/taxes need to be paid.
if you import a certain qtty, it can be considered for commercial use, and Customs will require a TISI license/permit from the importer together with the duties payment. If you don't have it, then think of 'tea money'...
Duties on helmets is normally 30% of helmet price+transport fm HK.
If your shipper can provide a certificate of origin FORM D for goods originated from China, then the duties fall to 0%.
For the helmets made in Japan, it is 5~10%.
in any case, you will have to pay the VAT @ 7% on hemet price+transport+duties