Are you thinking if one might say けっしょいます, けっしょって or something like that? Then definitely no, 結晶 itself is a simple noun that never conjugates. (A crystal maniac might try coining such a verb using this pattern as a joke, but I have never heard such a verb.) 結晶する is still ungrammatical. If you want
to say "to crystallize", it's 結晶化する (works transitively and intransitively), where 化 (か) is a suffix like -ization.
"結晶化!" is too long as a "transformation call", so 結晶 is a reasonable choice for that. Just because it's used similarly to "変身!" does not mean it's a verb. Shouting a pure noun is not strange even in English (e.g., "Attention!", "Silence!")