The easiest way to add drawing scales that you want available for all drawings is is to use the SCALELISTEDIT command (on the Annotate ribbon tab, Annotation Scaling panel, Scale List tool; or on the Drawing Window Status Bar, use the Annotation Scale control and select the Custom item on the popup list) to add these scales to your drawing template files. This will not, of course, affect any existing drawing files that you have.
"Vanilla" AutoCAD® offers the ability to customize the default scale list, with separate lists for Metric- and Imperial-unit drawings, by selecting the Default Scale List button in the Options dialog, on the User Preferences tab.
AutoCAD®Architecture, on the other hand, does not offer the ability to edit a default scale list,
In order to avoid having to add the additional scale(s) manually to existing files, you can start a new file from your edited template file. Set one of the added scales current, and add an annotative MText object. If you have more than one added scale, select the MText object, right click, and choose Annotative Object Scale > Add/Delete Scales from the context menu.