You can save a new email message in Outlook and copy and paste the content of your .txt file to the body of the email and save it. This will make a copy inside your .pst file. There also exist utilities that can help you with the very same thing, as they typically not only convert email messages but text files as well.
Convert Simple text file to Microsoft Outlook personal storage table.
We found one software available in the database for this conversion.
Microsoft Windows software - convert txt to pst on Windows
A simple converter for Email to Outlook files
Convert from txt file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.