Japanese bureaucracy provides for two names, which may be written with kanji, hiragana, or katakana (or any combination thereof) as you see fit. Foreign characters are not permitted. Neither are punctuation marks.
She can have the name smushed all together on official documents (山田スミス or スミス山田 for example) and in daily life and casual use write it however she darned well pleases (山田-スミス or スミス・山田 for example).
This is also the standard practice followed but hose of us with children we wish to have two given names (first and middle names), except that the smushing together happens in the given name box instead of the family name box.
I bought surname hyphenating was mostly a European thing...