If the records are already in a file, your best bet may be to edit the existing zone file directly (usually in the
%SystemDir%\System32\dns folder). Open the file for the zone in question and paste the records in at the bottom of the file. Ensure the record format is the same as existing records (for example, it's simply
CNAME rather than
Alias(CNAME) and the fully-qualified alias name (on the right) should have a period at the end to designate it as a fully-qualified domain name.
Once your edits are complete, load the DNS control panel, find the domain, right-click and select
Reload to have it read the file back into memory with the changes.
Note: Don't forget to make a backup of the original file before you begin editing, and don't forget to increment the serial number so any secondary servers will know to pull a zone transfer. This assumes the zone was set up to use a file (standard) rather than some other data source for its zone information.
DNSCMD tool can be used to add records to a zone programmatically through a batch file if needed, but for 50 records in one zone it's probably easier to just paste them in directly rather than trying to convert it to a batch file.