Yes, it is called AutoCorrect.
Automated Boilerplate Using Microsoft Word
A shortcut for this is on the Insert Symbols dialog.
Clicking on the AutoCorrect Button gives you the dialog to add the symbol.
I would recommend using a name that would not be typed by accident. Here the proposed name is "\rho."
Note, though, that most (if not all) Greek letters are already in the Math AutoCorrect list in both Upper and lower case. To use those in a regular document you need to check the box to use them outside of equations (the math area).