It sounds like you're looking either for what Unicode calls "Combining
Double Inverted Breve" 0361, or else for the usual phonetic character
for the "er" sound 025A.
If you only need it once in a while, you can type the four-digit
Unicode code (on the regular keyboard) followed by Alt+X.
To make the first one, type "e" then the Combining Double Inverted
Breve then the "r."
If you're going to need this a lot, then within Insert Symbol, for the
first one go to the "Combining Diacritical Marks" area, locate the
character on the grid, and assign a Keyboard Shortcut using the button
at the bottom. You'll find the second one under "IPA Extensions."