You can seed but need to know what variety you have in place already in order to keep the same leaf texture throughout the lawn. Zenith and Compadre are seeded variants of zoysia that have very good germination rates. You do need to remember to plant very shallow and keep watered. In our warm climate the seeds germinate within a week to 10 days. They are slow growing, taking about 60 days until they begin to 'run'.
Plugs or sod also do well. I have found that in established lawns the zoysia root structure will sprout new growth within a week after sodcutting and grow/fill in well within another 2 months with adequate fertilization and water.
If sod is available you are better off going that route.
Zoysia all the way, all the time here on Guam. $5 - $7 per square foot installed.