Syncopate surely is a word, for only words have definitions, and all definitions have words (it's just a matter of finding the right ones).

To syncopate is to alter a rhythm so as to make it a syncopated rhythm (ah - the lovliness of grammar). This can be done by making some notes longer than they were, or by adding extra notes or rests. That is, to syncopate is to obscure the bar line.

This action of transformation can be done during the compositional process, or during an actual composition, depending on the effect desired.