You can either:
A. Copy the PDS members to unique sequential datasets with the .TXT entension added to the dataset name. Then, you can mput * all of those datasets.
B. Put each member one-at-a-time to the remote filename with an appropriate file extension added to the filename.
C. Do what you've already done, and then run a script command on the remote server to rename all of the files in that folder with the additional .txt file extension.