Is it possible to create a special character and add it to special characters? I have to type up papers with medical abbreviations. 4 common abbreviations are ā, p, c, s (also with a line over them like the ā). Right now I have to use a text box with the line in it and place that over the letters p, c, and s, set the txt box to be in the background but selectable.  Then every time I edit the document the line moves separately from the letter and you can imagine what a pain it is.  I'd like to create these characters, add them to special characters and then insert from there.  

Does anyone know of a way to do this via special characters or know of another way to accomplish the same thing?
You can add a line over p, c, s either with the US Extended keyboard layout (base letter then option/alt + a) or the Character Viewer (double click on unicode 0304 after the base letter).  Best just copy them from here:

p̄
c̄
s̄

You may need to try different fonts to get them to display properly