No and yes!
During "Save As" the created extension is defined by UltraEdit by the file type below the name field. If you select *.txt, it will be .txt. The current file type setting is always stored in the INI file of UltraEdit. So if you select once .txt and never change it, .txt will be used always for new file.
You can modify the list of file types at Advanced - Configuration - File Types. Bring file type .txt to top of the list. Of course, if you delete all file types except *.txt, new files will be always saved as .txt. But this "solution" is not very good, because opening files with different file extension then cannot be done easily anymore with the standard file open dialog.