Вам нужно sep=","
.
Непосредственно из справки для write.zoo
:
read.zoo and write.zoo are convenience functions for reading and writing "zoo"
series from/to text files. They are convenience interfaces to read.table and
write.table, respectively.
... further arguments passed to read.table, write.table, or read.zoo, respectively.
Следовательно, это должно работать:
write.zoo(dbt,file="export.csv",index.name="okay",row.names=FALSE,col.names=TRUE,sep=",")