Current product REST capabilities are limited to creation of one user per REST call for user creation.
The best case for repetitive REST calls that can only be done one at a time is via scripting. The script calls curl statements for a set of values preset in a file used by the script.
A simple, and not syntactically correct, sample of what needs to be done is something like the following.
for i in $userNames; do
curl --header "Content-Type: application/xml" --request PUT --data '<user><name>$i</name><description>$i</description><enabled>true</enabled><removable>true</removable><timezone>UTC</timezone><culture>en-US</culture></user>' --url http://$DAhostname:$port/rest/........
Where the inputFile has a username on each line. It can get as complex as scripting skills allow.