I know that you can use Alt plus a series of number pad numbers to add special characters (see here), but that doesn't seem to work with the standard number keys (above the keyboard), and the Lenovo T450S does not not have standard setting to allow for a numpad. What can I do to accommodate this lack of numpad?
On laptops you can usually activate the Number Lock (NumLk or something similar to that) and that will convert some of the keyboard keys to a number pad. That should allow you to use the alt+code method to enter special characters. See http://www.top-password.com/blog/wp-content/uploads/2014/07/numlock.jpg for an example of the keys that are converted.
In some keyboards you may need to use a combination of the Fn key with another key to activate/deactivate NumLock. Make sure to deactivate it after entering the character, or you will be getting numbers when typing some letters.
Not extremely practical, I know, but should work.
Another method would be to use the Windows Character Map to find and use those characters.
Some of the characters can be input using a combination of the Left-Alt (or AltGr) key and other keyboard keys, depending on how your keyboard is configured (if you type in English with a standard US keyboard, you can try setting your keyboard layout to "US-International".
And, of course, if the characters you are trying to type are accented charaters, another method would be to add a language that uses such characters as an Input Method.
Hope it helps. If this isn't helpful enough, it may help us help you if you mention which character(s) you are trying to input.